tstest/tailmac: add customized macOS virtualization tooling (#13146)

updates tailcale/corp#22371

Adds custom macOS vm tooling.  See the README for
the general gist, but this will spin up VMs with unixgram
capable network interfaces listening to a named socket,
and with a virtio socket device for host-guest communication.

We can add other devices like consoles, serial, etc as needed.

The whole things is buildable with a single make command, and
everything is controllable via the command line using the TailMac
utility.

This should all be generally functional but takes a few shortcuts
with error handling and the like.  The virtio socket device support
has not been tested and may require some refinement.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

tstest/natlab/machost/README.md

@@ -0,0 +1,84 @@
+# macOS VM's for tstest and natlab
+
+## Building
+
+```
+%make all
+```
+
+Will build both the TailMac and the VMHost app.  You will need a developer account.  The default bundle identifiers
+default to tailscale owned ids, so if you don't have (or aren't using) a tailscale dev account, you will need to change this.
+This should build automatically as long as you have a valid developer cert.  Signing is automatic.  The binaries both
+require proper entitlements, so they do need to be signed.
+
+There are separate recipes in the makefile to rebuild the individual components if needed.
+
+All binaries are copied to the bin directory.
+
+You can generally do all interactions via the TailMac command line util.
+
+## Locations
+
+Everything is persisted at ~/VM.bundle
+
+Each vm gets it's own directory under there.  
+
+RestoreImage.ipsw is used to build new VMs.  You may replace this manually if you wish.
+
+Individual parameters for each instance are saved in a json config file (config.json)
+
+## Installing
+
+### Default a parameters
+
+The default virtio socket device port is 51009
+The default server socket for the virtual network device is /tmp/qemu.sock
+The default memory size is 4Gb
+The default mac address for the socket based network is 5a:94:ef:e4:0c:ee
+The defualt mac address for normal ethernet is 5a:94:ef:e4:0c:ef
+
+All of these parameters are configurable.
+
+### Creating and managing VMs
+
+To create a new VM (this will grab a restore image if needed).  Restore images are large.  Installation takes a minute
+```
+TailMac create --id my_vm_id
+```
+
+To delete a new VM 
+```
+TailMac delete --id my_vm_id
+```
+
+To refresh an existing restore image:
+```
+TailMac refresh
+```
+
+To clone an existing vm (this will clone the mac and port as well)
+```
+TailMac clone --id old_vm_id --target-id new_vm_id
+```
+
+To reconfigure a vm with a specific mac and a virtio socket device port:
+```
+TailMac configure --id vm_id --mac 11:22:33:44:55:66 --port 12345 --ethermac 22:33:44:55:66:77 --mem 4000000000 --sock "/var/netdevice.sock"
+```
+
+## Running a VM
+
+MacHost is an app bundle, but the main binary behaves as a command line util.  You can invoke it
+thusly:
+
+```
+TailMac --id machine_1
+ ```
+
+ You may invoke multiple vms, but the limit on the number of concurrent instances is on the order of 2.
+
+ To stop a running VM (this is a fire and forget thing):
+
+ ```
+ TailMac stop --id machine_1
+ ```