Include qapi/error.h exactly where needed

This cleanup makes the number of objects depending on qapi/error.h
drop from 1910 (out of 4743) to 1612 in my "build everything" tree.

While there, separate #include from file comment with a blank line,
and drop a useless comment on why qemu/osdep.h is included first.

Backports commit e688df6bc4549f28534cdb001f168b8caae55b0c from qemu

qemu/cpus.c

@@ -24,10 +24,10 @@
 
 /* Modified for Unicorn Engine by Nguyen Anh Quynh, 2015 */
 
-/* Needed early for CONFIG_BSD etc. */
 #include "qemu/osdep.h"
 #include "qemu-common.h"
 #include "cpu.h"
+#include "qapi/error.h"
 #include "sysemu/sysemu.h"
 #include "exec/exec-all.h"
 

qemu/hw/i386/pc.c

@@ -30,6 +30,7 @@
 #include "sysemu/cpus.h"
 #include "sysemu/sysemu.h"
 #include "target/i386/topology.h"
+#include "qapi/error.h"
 #include "qapi-visit.h"
 
 

qemu/hw/i386/pc_piix.c

@@ -25,6 +25,7 @@
 
 #include "qemu/osdep.h"
 #include "cpu.h"
+#include "qapi/error.h"
 #include "hw/i386/pc.h"
 #include "hw/boards.h"
 #include "exec/address-spaces.h"

qemu/qobject/qnum.c

@@ -13,7 +13,6 @@
  */
 
 #include "qemu/osdep.h"
-#include "qapi/error.h"
 #include "qapi/qmp/qnum.h"
 #include "qapi/qmp/qobject.h"
 #include "qemu-common.h"

qemu/scripts/qapi-event.py

@@ -206,6 +206,7 @@ fdef.write(mcgen('''
 #include "qemu-common.h"
 #include "%(prefix)sqapi-event.h"
 #include "%(prefix)sqapi-visit.h"
+#include "qapi/error.h"
 #include "qapi/qobject-output-visitor.h"
 #include "qapi/qmp-event.h"
 
@@ -213,7 +214,6 @@ fdef.write(mcgen('''
                  prefix=prefix))
 
 fdecl.write(mcgen('''
-#include "qapi/error.h"
 #include "qapi/qmp/qdict.h"
 #include "%(prefix)sqapi-types.h"
 

qemu/target/i386/cpu.c

@@ -19,13 +19,13 @@
 
 #include "qemu/osdep.h"
 #include "qemu/cutils.h"
-#include "qapi/error.h"
 #include "unicorn/platform.h"
 
 #include "cpu.h"
 #include "exec/exec-all.h"
 #include "sysemu/cpus.h"
 
+#include "qapi/error.h"
 #include "qapi/qmp/qerror.h"
 
 #include "qapi-visit.h"

qemu/vl.c

@@ -26,6 +26,7 @@
 /* By Nguyen Anh Quynh, 2015 */
 
 #include "qemu/osdep.h"
+#include "qapi/error.h"
 #include "qemu/cutils.h"
 #include "hw/boards.h"  // MachineClass
 #include "sysemu/sysemu.h"