OvmfPkg: QemuVideoDxe: work around misreported QXL framebuffer size

When setting up the list of GOP modes offered on QEMU's stdvga ("VGA") and
QXL ("qxl-vga") video devices, QemuVideoBochsModeSetup() filters those
modes against the available framebuffer size. (Refer to SVN r15288 / git
commit ec88061e.)

The VBE_DISPI_INDEX_VIDEO_MEMORY_64K register of both stdvga and QXL is
supposed to report the size of the drawable, VGA-compatibility
framebuffer. Instead, up to and including qemu-2.1, this register actually
reports the full video RAM (PCI BAR 0) size.

In case of stdvga, this happens to be correct, because on that card the
full PCI BAR 0 is usable for drawing; there is no difference between
"drawable framebuffer size" and "video RAM (PCI BAR 0) size".

However, on the QXL card, only an initial portion of the video RAM is
suitable for drawing, as compatibility framebuffer; and the value
currently reported by VBE_DISPI_INDEX_VIDEO_MEMORY_64K overshoots the
valid size. Beyond the drawable range, the video RAM contains buffers and
structures for the QXL guest-host protocol.

Luckily, the size of the drawable QXL framebuffer can also be read from a
register in the QXL ROM BAR (PCI BAR 2), so let's retrieve it from there.

Without this fix, OVMF offers too large resolutions on the QXL card (up to
the full size of the video RAM). If a GOP client selects such a resolution
and draws into the video RAM past the compatibility segment, then the
guest corrupts its communication structures (which is invalid guest
behavior).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15978 6f19259b-4bc3-4df7-8a09-765794883524

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