bhyve/snapshot: drop mkdir when creating the unix domain socket

[rew]

Summary

Add /var/run/bhyve/ to BSD.var.dist so we don't have to call mkdir when
creating the unix domain socket for a given bhyve vm.

The path to the unix domain socket for a bhyve vm will now be
/var/run/bhyve/vmname instead of /var/run/bhyve/checkpoint/vmname

Move BHYVE_RUN_DIR from snapshot.c to snapshot.h so it can be shared
to bhyvectl(8).

This review is a subset of (but does not depend on) changes found at
https://reviews.freebsd.org/D28199.