Probe for QEMU machine types

Probe for QEMU machine types

Currently we hardcode the QEMU machine types. We should really just
parse the output of 'qemu -M ?' so the lists don't get out of sync.

xenner doesn't support '-M ?', so we still need to hardcode that.

The horrible (const char *const *) is removed in a subsequent patch.

* src/qemu_conf.c: kill the arch_info*machines tables, retain the
  hardcoded xenner machine type, add qemudProbeMachineTypes() to
  run and parse 'qemu -M ?' and use it in qemudCapsInitGuest()