Fix loss of the emulated keyboard on Xen PV HVM domains.

xen/xenbus/xenbusb.c:
    In xenbusb_probe_children(), do not modify the XenBus state of
    devices for which we have no PV driver support. An emulated device
    we do support may share this backend.  Hide the node from XenBus
    instead.

    This prevents closing the vkbd device, which Qemu's emulated keyboard
    device is using as the source for keyboard events.

    Tested with qemu-xen-traditional, qemu-xen and qemu stubdomains, all
    working as expected.

Submitted by:	Roger Pau Monne <roger.pau@citrix.com>
Reviewed by:	gibbs
MFC after:	1 week