Propagate the error status of /usr/sbin/bhyve out of the vmrun.sh script.
Clean up the /dev/vmm entry for this VM when vmrun.sh exits.

Reviewed by: neel
Phabric:     D767


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271496 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f