Move QEMUMachine typedef to qemu/typedefs.h

As reported in commit 9c06a1f, 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>

include/hw/boards.h

@@ -6,8 +6,6 @@
 #include "sysemu/blockdev.h"
 #include "hw/qdev.h"
 
-typedef struct QEMUMachine QEMUMachine;
-
 typedef struct QEMUMachineInitArgs {
     const QEMUMachine *machine;
     ram_addr_t ram_size;

include/qemu/typedefs.h

@@ -30,6 +30,7 @@ typedef struct MemoryListener MemoryListener;
 
 typedef struct MemoryMappingList MemoryMappingList;
 
+typedef struct QEMUMachine QEMUMachine;
 typedef struct NICInfo NICInfo;
 typedef struct HCIInfo HCIInfo;
 typedef struct AudioState AudioState;