xen: groundwork for xen support (Gerd Hoffmann)

- configure script and build system changes.
- wind up new machine type.
- add -xen-* command line options.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7219 c046a42c-6fe2-441c-8c8c-71466251a162