vmrun.sh: Add arm64 support

For now, we enumerate disk devices before network devices. This is to
work around a problem wherein u-boot remaps BARs during boot in a way
that bhyve does not handle. Some discussion and experiments suggest
that this can be handled by having bhyve not map BARs during boot on
arm64; until a solution is implemented, however, this workaround is
sufficient for simple usage and doesn't have any real downsides.

The console and bootrom are specified slightly differently versus amd64,
and a few of vmrun.sh's command-line options are amd64-only.

Reviewed by: corvink, jhb
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D44933