vfio: Introduce new files for VFIORegion definitions and declarations

vfio: Introduce new files for VFIORegion definitions and declarations

Gather all VFIORegion related declarations and definitions into their
own files to reduce exposure of VFIO internals in "hw/vfio/vfio-common.h".
They were introduced for 'vfio-platform' support in commits
db0da029a185 ("vfio: Generalize region support") and a664477db8da
("hw/vfio/pci: Introduce VFIORegion").

To be noted that the 'vfio-platform' devices have been deprecated and
will be removed in QEMU 10.2. Until then, make the declarations
available externally for 'sysbus-fdt.c'.

Cc: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-12-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>