Call "vmctl stop" on each VM at shutdown, for OpenBSD guests this means they

Call "vmctl stop" on each VM at shutdown, for OpenBSD guests this means they
are signalled to shutdown cleanly. Wait for each to finish to avoid too much
busy work at once; this may need revising if it turns out to be too slow with
a larger number of VMs (e.g. signal/delay/signal/delay/... then wait for
shutdowns), but let's avoid making it more complex unless we know it's needed.

Based on a diff from abieber@, discussed with mlarkin@ aja@ rpe@, ok rpe