ArmVirtPkg: remove QemuVideoDxe from ArmVirtQemu and ArmVirtQemuKernel

ArmVirtPkg: remove QemuVideoDxe from ArmVirtQemu and ArmVirtQemuKernel

One of the reasons for introducing virtio-gpu support to OvmfPkg and
ArmVirtpkg was the fact that under KVM virtualization on ARM, the
legacy VGA cannot be used reliably. This is due to an implementation
detail of QEMU+KVM, which remaps cached host memory into the guest
address space as a framebuffer behind a PCI BAR. Given that the purpose
of a memory mapped framebuffer is its side effects, such BARs should
never be mapped cacheable in the guest, and the mismatched attributes
between host and guest result in a loss of coherency, visible as
corruption in the framebuffer image.

This issue does not occur under TCG emulation, nor did we expect it to
actually bring down the guest under KVM, and so it was deemed harmless
to keep support for the VGA device as well. However, as it turns out,
the fact that the framebuffer BAR is mapped using device semantics by
default may result in unalignment faults when we use the ordinary string
copy routines on the contents. In theory, we could work around this by
remapping the BAR as write combining, but it appears the generic PCI
bus driver does not actually implement this.

So let's remove the QemuVideoDxe driver altogether. This may result
in loss of functionality for use cases that rely on the framebuffer
to be directly addressable (such as EFIFB), but given that this never
worked reliably under KVM in the first place, let's not let that stop
us from dropping support for it.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>