vfio/migration: Multifd device state transfer support - basic types

vfio/migration: Multifd device state transfer support - basic types

Add basic types and flags used by VFIO multifd device state transfer
support.

Since we'll be introducing a lot of multifd transfer specific code,
add a new file migration-multifd.c to home it, wired into main VFIO
migration code (migration.c) via migration-multifd.h header file.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/4eedd529e6617f80f3d6a66d7268a0db2bc173fa.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>