KVM: Split QEMUMachine typedef into separate header

Older gcc versions (such as the one in SLES11) get confused when you declare
a typedef on the same struct twice.

To work around that limitation, let's extract the QEMUMachine typedef into a
separate header file that is guarded by preprocessor duplicate include checks.

This fixes the following type of compile errors for me:

  In file included from vl.c:125:
  include/hw/xen/xen.h:39: error: redefinition of typedef "QEMUMachine"
  include/sysemu/kvm.h:155: error: previous declaration of "QEMUMachine" was here

Signed-off-by: Alexander Graf <agraf@suse.de>
