vfio: Introduce a new header file for internal migration services

vfio: Introduce a new header file for internal migration services

Gather all VFIO migration related declarations into
"vfio-migration-internal.h" to reduce exposure of VFIO internals in
"hw/vfio/vfio-common.h".

Cc: Kirti Wankhede <kwankhede@nvidia.com>
Cc: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Avihai Horon <avihaih@nvidia.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-7-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>