pcie_sriov: Register VFs after migration

pcie_sriov doesn't have code to restore its state after migration, but
igb, which uses pcie_sriov, naively claimed its migration capability.

Add code to register VFs after migration and fix igb migration.

Fixes: 3a977dee ("Intrdocue igb device emulation")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20250116-reuse-v20-11-7cb370606368@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>