This patch eliminate the global variables in libxenctrl (used for
logging and error reporting).

Instead the information which was in the global variables is now in a
new xc_interface* opaque structure, which xc_interface open returns
instead of the raw file descriptor; furthermore, logging is done via
xentoollog.

There are three new parameters to xc_interface_open to control the
logging, but existing callers can just pass "0" for all three to get
the old behaviour.

All libxc callers have been adjusted accordingly.

Also update QEMU_TAG for corresponding qemu change.

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