qxl: set default values of vram*_size_mb to -1

The addition of those values caused a regression where not specifying
any value for the vram bar size would result in a 4096 _byte_ surface
area. This is ok for the windows driver but causes the X driver to be
unusable. Also, it's a regression. This patch returns the default
behavior of having a 64 megabyte vram BAR.

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>