CP-3296: conditionally-compile the new xenguest interface
CP-3296: conditionally-compile the new xenguest interface

In the XCP xen patchqueue I've added the export
  XENGUEST_HAS_HVM_BUILD_ARGS
to the patch which backports xen-unstable.hg c/s 7e1fd8012ca3

The new interface should also be present in xen-4.2 onwards so we also
use it if
  __XEN_LATEST_INTERFACE_VERSION__ >= 0x00040200

Hopefully this will compile and work on:
  xen-4.2 (not tested)
  xen-unstable (not tested)
  xen-4.1 (tested, OK)
  XCP with xen-4.1 + backports (tested, OK)

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

djs55/squeezed,djs55/xcp-rrdd,johnelse/xcp-rrdd,simonjbeaumont/xcp-rrdd,koushikcgit/xcp-rrdd,robhoes/squeezed,koushikcgit/xcp-rrdd,simonjbeaumont/xcp-rrdd,koushikcgit/xcp-rrdd,johnelse/xcp-rrdd,sharady/xcp-networkd,djs55/xcp-rrdd,djs55/xcp-networkd,koushikcgit/xcp-networkd,koushikcgit/xcp-networkd,djs55/xcp-networkd,sharady/xcp-networkd