vga: Silence bogus gcc warning about uninitialized variables

Some gcc versions do not properly detect that all possible cases are
covered and base and size are always initialized. Please gcc by defining
a pseudo default case.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
