QXL: fix setting ram and vram values for QEMU QXL device

QXL: fix setting ram and vram values for QEMU QXL device

QEMU has two different type of QXL display device. The first "qxl-vga"
is for primary video device and second "qxl" is for secondary video
device.

There are also two different ways how to specify those devices on qemu
command line, the first one and obsolete is using "-vga" option and the
current new one is using "-device" option. The "-vga" could be used only
to setup primary video device, so the "-vga qxl" equal to
"-device qxl-vga". Unfortunately the "-vga qxl" doesn't support setting
additional parameters for the device and "-global" option must be used
for this purpose. It's mandatory to use "-global qxl-vga...." to set the
parameters of primary video device previously defined with "-vga qxl".

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1076098

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>