Create 3 new function refactored from previous list ones and
exports them internally to the driver

* src/libxl/libxl_conf.c src/libxl/libxl_conf.h: create libxlMakeDisk,
  libxlMakeNic libxlMakeVfb out of the exsting static List functions
  and exports them

The cur_vcpus member of struct libxl_domain_build_info was incorrectly
initialized to the number of vcpus, when it should have been interpreted
as a bitmap, where bit X corresponds to online/offline status of vcpuX.

To complicate matters, cur_vcpus is an int, so only 32 vcpus can be
set online.  Add a check to ensure vcpus does not exceed this limit.

V2: Eric Blake noted a compilation pitfal when '1 << 32' on an int.
    Account for vcpus == 32.

libxl accepts hpet configuration in its domain info struct.  Parse the
domain definition's <clock> element in order to set the value.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Apologies from Eric Blake, for mistakenly committing the broken
intermediate version.

libxl accepts hpet configuration in its domain info struct.  Parse the
domain definition's <clock> element in order to set the value.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

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

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