libxc: convert evtchn interfaces to use an opaque handle type

libxc: convert evtchn interfaces to use an opaque handle type

This makes the interface consistent with the changes made to the main
interface in 21483:779c0ef9682c.

Also fix some references to "struct xc_interface" which should have
been simply "xc_interface" in tools/xenpaging, and update QEMU_TAG to
pull in the corresponding qemu change.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>