qemu: map "virtio" video model to "virt" machtype correctly (arm/aarch64)

qemu: map "virtio" video model to "virt" machtype correctly (arm/aarch64)

Most of QEMU's PCI display device models, such as:

  libvirt video/model/@type  QEMU -device
  -------------------------  ------------
  cirrus                     cirrus-vga
  vga                        VGA
  qxl                        qxl-vga
  virtio                     virtio-vga

come with a linear framebuffer (sometimes called "VGA compatibility
framebuffer"). This linear framebuffer lives in one of the PCI device's
MMIO BARs, and allows guest code (primarily: firmware drivers, and
non-accelerated OS drivers) to display graphics with direct memory access.

Due to architectural reasons on aarch64/KVM hosts, this kind of
framebuffer doesn't / can't work in

  qemu-system-(arm|aarch64) -M virt

machines. Cache coherency issues guarantee a corrupted / unusable display.
The problem has been researched by several people, including kvm-arm
maintainers, and it's been decided that the best way (practically the only
way) to have boot time graphics for such guests is to consolidate on
QEMU's "virtio-gpu-pci" device.

>From <https://bugzilla.redhat.com/show_bug.cgi?id=1195176>, libvirt
supports

  <devices>
    <video>
      <model type='virtio'/>
    </video>
  </devices>

but libvirt unconditionally maps @type='virtio' to QEMU's "virtio-vga"
device model. (See the qemuBuildDeviceVideoStr() function and the
"qemuDeviceVideo" enum impl.)

According to the above, this is not right for the "virt" machine type; the
qemu-system-(arm|aarch64) binaries don't even recognize the "virtio-vga"
device model (justifiedly). Whereas "virtio-gpu-pci", which is a pure
virtio device without a compatibility framebuffer, is available, and works
fine.

(The ArmVirtQemu ("AAVMF") platform of edk2 -- that is, the UEFI firmware
for "virt" -- supports "virtio-gpu-pci", as of upstream commit
3ef3209d3028. See
<https://tianocore.acgmultimedia.com/show_bug.cgi?id=66>.)

Override the default mapping of "virtio", from "virtio-vga" to
"virtio-gpu-pci", if qemuDomainMachineIsVirt() evaluates to true.

Cc: Andrea Bolognani <abologna@redhat.com>
Cc: Drew Jones <drjones@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Martin Kletzander <mkletzan@redhat.com>
Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1372901
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Martin Kletzander <mkletzan@redhat.com>