vfio: fix possible use after free of vfio group

The vfio group should be released after
the vfio_group_try_dissolve_container call.
The code should not rely on someone else to hold
a reference on the group.

Signed-off-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>