libxl: use LIBXL_API_VERSION 0x040200

libxl: use LIBXL_API_VERSION 0x040200

To ensure the libvirt libxl driver will build with future versions
of Xen where the libxl API may change in incompatible ways,
explicitly use LIBXL_API_VERSION 0x040200. The libxl driver
does use new libxl APIs that have been added since Xen 4.2, but
currently it does not make use of any changes made to existing
APIs such as libxl_domain_create_restore or libxl_set_vcpuaffinity.
The version can be bumped if/when the libxl driver consumes the
changed APIs.

Further details can be found in the following discussion thread

https://www.redhat.com/archives/libvir-list/2016-April/msg00178.html
Signed-off-by: Jim Fehlig <jfehlig@suse.com>