qemu-xen: add vkbd support for PV on HVM guests

qemu-xen: add vkbd support for PV on HVM guests

Register the vkbd backend even when running as device emulator for HVM
guests: it is useful because it doesn't need a frequent timer like usb.

Check whether the XenInput DisplayState has been set in the initialise
state, rather than the input state.
In case the DisplayState hasn't been set and there is no vfb for this
domain, then set the XenInput DisplayState to the default one.

An equivalent patch has already been committed in upstream qemu.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>