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(default qxl_vram_size=33554432) and
    qxl_vgamem_mb=16 (default qxl_vgamem_mb=8). 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>