vfio: Fix unregister SaveVMHandler in vfio_migration_finalize

vfio: Fix unregister SaveVMHandler in vfio_migration_finalize

In the vfio_migration_init(), the SaveVMHandler is registered for
VFIO device. But it lacks the operation of 'unregister'. It will
lead to 'Segmentation fault (core dumped)' in
qemu_savevm_state_setup(), if performing live migration after a
VFIO device is hot deleted.

Fixes: 7c2f5f75f94 (vfio: Register SaveVMHandlers for VFIO device)
Reported-by: Qixin Gan <ganqixin@huawei.com>
Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
Message-Id: <20210527123101.289-1-jiangkunkun@huawei.com>
Reviewed by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
(cherry picked from commit 22fca190e25b10761925bb1eeadeda07aabf3c26)
Signed-off-by: Michael Roth <michael.roth@amd.com>