Add libxenlight driver

Add a new xen driver based on libxenlight [1], which is the primary
toolstack starting with Xen 4.1.0.  The driver is stateful and runs
privileged only.

Like the existing xen-unified driver, the libxenlight driver is
accessed with xen:// URI.  Driver selection is based on the status
of xend.  If xend is running, the libxenlight driver will not load
and xen:// connections are handled by xen-unified.  If xend is not
running *and* the libxenlight driver is available, xen://
connections are deferred to the libxenlight driver.

V6:
 - Address several code style issues noted by Daniel Veillard
 - Make drive work with xen:/// URI
 - Hold domain object reference while domain is injected in
   libvirt event loop.  Race found and fixed by Markus Groß.

V5:
 - Ensure events are unregistered when domain private data
   is destroyed.  Discovered and fixed by Markus Groß.

V4:
 - Handle restart of libvirtd, reconnecting to previously
   started domains
 - Rebased to current master
 - Tested against Xen 4.1 RC7-pre (c/s 22961:c5d121fd35c0)

V3:
  - Reserve vnc port within driver when autoport=yes

V2:
  - Update to Xen 4.1 RC6-pre (c/s 22940:5a4710640f81)
  - Rebased to current master
  - Plug memory leaks found by Stefano Stabellini and valgrind
  - Handle SHUTDOWN_crash domain death event

[1] http://lists.xensource.com/archives/html/xen-devel/2009-11/msg00436.html

daemon/Makefile.am

@@ -106,6 +106,10 @@ if WITH_LXC
     libvirtd_LDADD += ../src/libvirt_driver_lxc.la
 endif
 
+if WITH_LIBXL
+    libvirtd_LDADD += ../src/libvirt_driver_libxl.la
+endif
+
 if WITH_UML
     libvirtd_LDADD += ../src/libvirt_driver_uml.la
 endif

daemon/libvirtd.c

@@ -81,6 +81,9 @@
 # ifdef WITH_LXC
 #  include "lxc/lxc_driver.h"
 # endif
+# ifdef WITH_LIBXL
+#  include "libxl/libxl_driver.h"
+# endif
 # ifdef WITH_UML
 #  include "uml/uml_driver.h"
 # endif
@@ -943,6 +946,9 @@ static struct qemud_server *qemudInitialize(void) {
 # ifdef WITH_NWFILTER
     nwfilterRegister();
 # endif
+# ifdef WITH_LIBXL
+    libxlRegister();
+# endif
 # ifdef WITH_QEMU
     qemuRegister();
 # endif