Log Message:
Add options for qemu and xen, and disable all by default.

Before, anita depended on qemu0 or qemu.  Now, it has options for
qemu-0, qemu (currently 2) and xen (currently 4.1).  By default, all
are off because all of them are quite large and they need only be
present at run time.

Discussed with gson@, who did not object.