vfio/virtio: Enable live migration once VIRTIO_PCI was configured

vfio/virtio: Enable live migration once VIRTIO_PCI was configured

Now that the driver supports live migration, only the legacy IO
functionality depends on config VIRTIO_PCI_ADMIN_LEGACY.

As part of that we introduce a bool configuration option as a sub menu
under the driver's main live migration feature named
VIRTIO_VFIO_PCI_ADMIN_LEGACY, to control the legacy IO functionality.

This will let users configuring the kernel, know which features from the
description might be available in the resulting driver.

As of that, move the legacy IO into a separate file to be compiled only
once CONFIG_VIRTIO_VFIO_PCI_ADMIN_LEGACY was configured and let the live
migration depends only on VIRTIO_PCI.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20241113115200.209269-8-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>