OvmfPkg: change qemu default resolution to 1280x800

OvmfPkg: change qemu default resolution to 1280x800

ovmf default display resolution is 800x600.  This is rather small for
modern guests.  qemu used 1024x768 as default for a long time and
switched the to 1280x800 recently[1] for the upcoming 7.0 release.

This patch brings ovmf in sync with the recent qemu update and likewise
switches the default to 1280x800.

[1] https://gitlab.com/qemu-project/qemu/-/commit/de72c4b7cdf6ec18bfe9fe714aa96e48db6fd895

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>