OvmfPkg: set video resolution of text setup to 640x480

OvmfPkg: set video resolution of text setup to 640x480

On a physical screen such a low graphics resolution would lead to huge
glyphs (the text resolution is 80x25, centered, with 8x19 pixel glyphs).
But in a virtual machine it just saves screen real estate on the client,
by removing the black bands.

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@16311 6f19259b-4bc3-4df7-8a09-765794883524