vfio/mdpy: Fix memory leak of object mdev_state->vconfig

In the case where the call to vfio_register_group_dev fails the error
return path kfree's mdev_state but not mdev_state->vconfig. Fix this
by kfree'ing mdev_state->vconfig before returning.

Addresses-Coverity: ("Resource leak")
Fixes: 437e4136

 ("vfio/mdpy: Convert to use vfio_register_group_dev()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210622183710.28954-1-colin.king@canonical.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>