OvmfPkg/VirtioGpuDxe: provide functions for sending VirtIo GPU commands

OvmfPkg/VirtioGpuDxe: provide functions for sending VirtIo GPU commands

In this patch we add a "workhorse" function called VirtioGpuSendCommand(),
and implement seven simple RPCs atop, for the command types listed in
"OvmfPkg/Include/IndustryStandard/VirtioGpu.h".

These functions will be called by our EFI_GRAPHICS_OUTPUT_PROTOCOL
implementation.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://tianocore.acgmultimedia.com/show_bug.cgi?id=66
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>