virttest.qemu_vm: add support set VGA vram ram vgamem

For qxl device, we can set global default by params like, qxl_vram_size=67108864 and
qxl_vgamem_mb=16 . For 'std' VGA set vgamem_mb like vga_vgamem_mb=16, 'cirrus'
VGA set like, cirrus-vga_vgamem_mb =16.

Signed-off-by: Xu Tian <xutian@redhat.com>