Arm64 vmrun.sh for bhyve

[bsdjhb]

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This let me run a VM similar to what I use for amd64 as a single line:

name="installer"
tap="tap0"
sudo sh /path/to/vmrun.sh -c 1 -t ${tap} -m 4G -d /path/to/disks/${name}.img "$@" ${name}

I have not figured out the right incantation of bsd.files.mk magic to install the file. It would be simplest to add a Makefile in share/examples/bhyve I think. For upstream I would probably rename the existing vmrun.sh to vmrun.amd64.sh so it always had MACHINE_ARCH in the source file name. Not sure I wanted to do that in this downstream PR yet though.

[jrtc27]

I'd rather have this in a port so we can update it easily (e.g. as firmware gets more featureful)