vfio/pci: Stub vfio_pci_vga_rw when !CONFIG_VFIO_PCI_VGA

vfio/pci: Stub vfio_pci_vga_rw when !CONFIG_VFIO_PCI_VGA

Resolve build errors reported against UML build for undefined
ioport_map() and ioport_unmap() functions.  Without this config
option a device cannot have vfio_pci_core_device.has_vga set,
so the existing function would always return -EINVAL anyway.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20220123125737.2658758-1-geert@linux-m68k.org
Link: https://lore.kernel.org/r/164306582968.3758255.15192949639574660648.stgit@omen
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>