vfio: Introduce helper vfio_pci_from_vfio_device()

vfio: Introduce helper vfio_pci_from_vfio_device()

Introduce helper vfio_pci_from_vfio_device() to transform from VFIODevice
to VFIOPCIDevice, also to hide low level VFIO_DEVICE_TYPE_PCI type check.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250822064101.123526-5-zhenzhong.duan@intel.com
[ clg: Added documentation ]
Signed-off-by: Cédric Le Goater <clg@redhat.com>