qemu_process: detect updated video ram size values from QEMU
    
    QEMU internally updates the size of video memory if the domain XML had
    provided too low memory size or there are some dependencies for a QXL
    devices 'vgamem' and 'ram' size. We need to know about the changes and
    store them into the status XML to not break migration or managedsave
    through different libvirt versions.
    
    The values would be loaded only if the "vgamem_mb" property exists for
    the device.  The presence of the "vgamem_mb" also tells that the
    "ram_size" and "vram_size" exists for QXL devices.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>