qxl: change the default value for vgamem_mb to 16 MiB
    
    The default value should be 16 MiB instead of 8 MiB. Only really old
    version of upstream QEMU used the 8 MiB as default for vga framebuffer.
    
    Without this change if you update your libvirt where we introduced the
    "vgamem" attribute for QXL video device the value will be set to 8 MiB,
    but previously your guest had 16 MiB because we didn't pass any value to
    QEMU command line which means QEMU used its own 16 MiB as default.
    
    This will affect all users with guest's display resolution higher than
    1920x1080.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>