OvmfPkg/QemuVideoDxe: round up FrameBufferSize to full page

OvmfPkg/QemuVideoDxe: round up FrameBufferSize to full page

Guests do the same, because the framebuffer is mapped somewhere, which
obviously works with page granularity only.

When not rounding up to full page size we get messages like this one
(linux kernel):

    efifb: framebuffer at 0x80000000, using 1876k, total 1875k
                                            ^^^^^        ^^^^^
Also sysfb is confused and throws an error:

    sysfb: VRAM smaller than advertised

Cc: Phil Dennis-Jordan <phil@philjordan.eu>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: fix coding style]