migration/qemu-file.h: Move QEMUFile declarations here.

Move the QEMUFile-related declarations to migration/qemu-file.h,
from hw/hw.h to get closer to upstream. Note that this also
updates the implementation slightly. The end result is still
different from upstream in certain ways, that will be handled
in future patches.

Change-Id: I11719c4449040df13d9e6f62c55d642a0dd6434d
diff --git a/cpus.c b/cpus.c
index ce13a55..8e6c178 100644
--- a/cpus.c
+++ b/cpus.c
@@ -27,6 +27,7 @@
 #include "exec/exec-all.h"
 #include "monitor/monitor.h"
 #include "sysemu/sysemu.h"
+#include "exec/exec-all.h"
 #include "exec/gdbstub.h"
 #include "sysemu/dma.h"
 #include "sysemu/kvm.h"