qemu-command: introduce new vgamem attribute for QXL video device

Add attribute to set vgamem_mb parameter of QXL device for QEMU. This
value sets the size of VGA framebuffer for QXL device. Default value in
QEMU is 8MB so reuse it also in libvirt to not break things.

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



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