vfio: Destroy memory regions

Somehow this has been lurking for a while; we remove our subregions
from the base BAR and VGA region mappings, but we don't destroy them,
creating a leak and more serious problems when we try to migrate after
removing these devices.  Add the trivial bit of final cleanup to
remove these entirely.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
