vfio/mbochs: Use the new device life cycle helpers

and manage avail_mbytes inside @init/@release.

Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20220921104401.38898-8-kevin.tian@intel.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>