Provide alternative Xen backend infrastructure (Gerd Hoffman)

Provide alternative Xen backend infrastructure (Gerd Hoffman)

Description from Gerd's message:

    Subject: [PATCH] xen: backend driver core

    This patch adds infrastructure for xen backend drivers living in qemu,
    so drivers don't need to implement common stuff on their own.  It's
    mostly xenbus management stuff: some functions to access xentore,
    setting up xenstore watches, callbacks on device discovery and state
    changes, handle event channel, ...

In this patch, this is just additional machinery, which will be used
in the forthcoming change(s).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(Applied as-is.)