xen: drop support for Xen 4.1 and older.

xen: drop support for Xen 4.1 and older.

Xen 4.2 become unsupported upstream in 09/2015 (see
http://wiki.xen.org/wiki/Xen_Release_Features). However as far as the
interfaces provided by the toolstack libraries go 4.2 and 4.3 are
indistinguishable.

Therefore drop support for Xen 4.1 and earlier which removes a whole
pile of compatibility code which makes future work (to use stable
library interfaces provided by upstream) more difficult. In particular
all supported versions now use a pointer as a libxc handle (4.1 and
earlier used an integer, resulting in various shim layers).

Also Xen 4.2 was the first version of Xen to formally support upstream
QEMU (as a preview) so that makes sense as a cut-off now.

This change drops all the configure-y and resulting ifdefs in a mostly
mechanical way. A follow up will refactor wrappers which are now
unused.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
---
v2: Don't claim to support 4.1 for another 89 years in the commit
      message.
    Tweak message in configure.