OvmfPkg: QemuVideoDxe: filter BOCHS modes vs. available frame buffer size

OvmfPkg: QemuVideoDxe: filter BOCHS modes vs. available frame buffer size

In the next patch we'll add many new BOCHS modes, some of which require
large frame buffers.

The size of the QXL VGA compatibility framebuffer can be changed with the

  -global qxl-vga.vgamem_mb=$NUM_MB

QEMU option.

If $NUM_MB would exceed 32, then the following two QEMU options are
necessary instead:

  -global qxl-vga.vgamem_mb=$NUM_MB         \
  -global qxl-vga.ram_size_mb=$((NUM_MB*2))

because the compatibility framebuffer can't cover more than half of PCI
BAR #0. The latter defaults to 64MB in size, and is controlled by
"ram_size_mb".

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15288 6f19259b-4bc3-4df7-8a09-765794883524