As reported in commit 9c06a1f79f, xen.h is
not self-contained with regards to its use of QEMUMachine. Fix this.
Reported-by: Alexander Graf <agraf@suse.de>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>