vfio: Introduce a new header file for VFIOdisplay declarations

vfio: Introduce a new header file for VFIOdisplay declarations

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

Reviewed-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20250318095415.670319-8-clg@redhat.com
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-9-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>