Convert libxl driver to Xen 4.2

Convert libxl driver to Xen 4.2

Based on a patch originally authored by Daniel De Graaf

  http://lists.xen.org/archives/html/xen-devel/2012-05/msg00565.html

This patch converts the Xen libxl driver to support only Xen >= 4.2.
Support for Xen 4.1 libxl is dropped since that version of libxl is
designated 'technology preview' only and is incompatible with Xen 4.2
libxl.  Additionally, the default toolstack in Xen 4.1 is still xend,
for which libvirt has a stable, functional driver.
(cherry picked from commit dfa1e1dd5325f3a6bc8865312cde557044bf0b93)

Conflicts:
	src/libxl/libxl_conf.c - commit e5e8d5 not backported
	src/libxl/libxl_driver.c - commit 1c04f99 not backported