vfio/migration: Add VFIO migration pre-copy support

vfio/migration: Add VFIO migration pre-copy support

Pre-copy support allows the VFIO device data to be transferred while the
VM is running. This helps to accommodate VFIO devices that have a large
amount of data that needs to be transferred, and it can reduce migration
downtime.

Pre-copy support is optional in VFIO migration protocol v2.
Implement pre-copy of VFIO migration protocol v2 and use it for devices
that support it. Full description of it can be found in the following
Linux commit: 4db52602a607 ("vfio: Extend the device migration protocol
with PRE_COPY").

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>