qemu: add 'ramfb' attribute for mediated devices

qemu: add 'ramfb' attribute for mediated devices

The 'ramfb' attribute provides a framebuffer to the guest that can be
used as a boot display for the vgpu

For example, the following configuration can be used to provide a vgpu
with a boot display:

    <hostdev mode='subsystem' type='mdev' model='vfio-pci' display='on' ramfb='on'>
        <source>
            <address uuid='$UUID'/>
        </source>
    </hostdev>

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>