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@omenSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci_rdwr.c

@@ -288,6 +288,7 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 	return done;
 }
 
+#ifdef CONFIG_VFIO_PCI_VGA
 ssize_t vfio_pci_vga_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 			       size_t count, loff_t *ppos, bool iswrite)
 {
@@ -355,6 +356,7 @@ ssize_t vfio_pci_vga_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 
 	return done;
 }
+#endif
 
 static void vfio_pci_ioeventfd_do_write(struct vfio_pci_ioeventfd *ioeventfd,
 					bool test_mem)

include/linux/vfio_pci_core.h

@@ -159,8 +159,17 @@ extern ssize_t vfio_pci_config_rw(struct vfio_pci_core_device *vdev,
 extern ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 			       size_t count, loff_t *ppos, bool iswrite);
 
+#ifdef CONFIG_VFIO_PCI_VGA
 extern ssize_t vfio_pci_vga_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 			       size_t count, loff_t *ppos, bool iswrite);
+#else
+static inline ssize_t vfio_pci_vga_rw(struct vfio_pci_core_device *vdev,
+				      char __user *buf, size_t count,
+				      loff_t *ppos, bool iswrite)
+{
+	return -EINVAL;
+}
+#endif
 
 extern long vfio_pci_ioeventfd(struct vfio_pci_core_device *vdev, loff_t offset,
 			       uint64_t data, int count, int fd);