hw: move headers to include/

Many of these should be cleaned up with proper qdev-/QOM-ification.
Right now there are many catch-all headers in include/hw/ARCH depending
on cpu.h, and this makes it necessary to compile these files per-target.
However, fixing this does not belong in these patches.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/exec.c b/exec.c
index 786987a..fa1e0c3 100644
--- a/exec.c
+++ b/exec.c
@@ -31,7 +31,7 @@
 #include "hw/qdev.h"
 #include "qemu/osdep.h"
 #include "sysemu/kvm.h"
-#include "hw/xen.h"
+#include "hw/xen/xen.h"
 #include "qemu/timer.h"
 #include "qemu/config-file.h"
 #include "exec/memory.h"