xen: Support new libxc calls from xen unstable.

This patch updates the libxenctrl calls in Qemu to use the new interface,
otherwise Qemu wouldn't be able to build against new versions of the
library.

We check libxenctrl version in configure, from Xen 3.3.0 to Xen
unstable.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
