vfio/virtio: Enable support for virtio-block live migration

vfio/virtio: Enable support for virtio-block live migration

With a functional and tested backend for virtio-block live migration,
add the virtio-block device ID to the pci_device_id table.

Currently, the driver supports legacy IO functionality only for
virtio-net, and it is accounted for in specific parts of the code.

To enforce this limitation, an explicit check for virtio-net, has been
added in virtiovf_support_legacy_io(). Once a backend implements legacy
IO functionality for virtio-block, the necessary support will be added
to the driver, and this additional check should be removed.

The module description was updated accordingly.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20250302162723.82578-1-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>