Hook the virtual cpu functions to their libxenlight counterparts

* src/libxl/libxl_driver.c: implements libxlDomainSetVcpus,
  libxlDomainGetVcpus, libxlDomainSetVcpusFlags,
  libxlDomainGetVcpusFlags and libxlDomainPinVcpu

* src/libxl/libxl_conf.[ch] src/libxl/libxl_driver.[ch]: add authors
  after the licence template

* src/libxl/libxl_conf.h: add the necessary fields to the driver
  private structure
* src/libxl/libxl_driver.c: add lifecycle event support and entry
  points for event(de)register(any)

* src/libxl/libxl_driver.c: use ignore_value() in libxlDomainObjUnref
  and libxlCreateDomEvents

It is possible to set a migration speed limit when starting
migration. This new API allows the speed limit to be changed
on the fly to adjust to changing conditions

* src/driver.h, src/libvirt.c, src/libvirt_public.syms,
  include/libvirt/libvirt.h.in: Add virDomainMigrateSetMaxSpeed
* src/esx/esx_driver.c, src/lxc/lxc_driver.c,
  src/opennebula/one_driver.c, src/openvz/openvz_driver.c,
  src/phyp/phyp_driver.c, src/qemu/qemu_driver.c,
  src/remote/remote_driver.c, src/test/test_driver.c,
  src/uml/uml_driver.c, src/vbox/vbox_tmpl.c,
  src/vmware/vmware_driver.c, src/xen/xen_driver.c,
  src/libxl/libxl_driver.c: Stub new API

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