Config.mk: Fix (and, effectively, update) QEMU_TAG

In 952944f7 "QEMU_TAG update" my tag update script mangled the
machinery which sets QEMU_TRADITIONAL_REVISION, by replacing the first
assignment to QEMU_TRADITIONAL_REVISION it found rather than the one
which ought to have been replaced.

The result was that:
 * From that commit on, QEMU_TAG was no longer honoured although
   QEMU_TRADITIONAL_REVISION still was
 * That particular update to QEMU_TRADITIONAL_REVISION's default
   value was effective
 * The next attempt to update QEMU_TRADITIONAL_REVISION, in
   1fc3aeb3 "libxl: use new QEMU xenstore protocol" was totally
   ineffective.

Fix this by restoring the transfer from QEMU_TAG.  The effects are:
 * Once more, honour QEMU_TAG.
 * Belatedly apply the qemu-trad change part of "libxl: use new QEMU
   xenstore protocol.

(I have also fixed my script to not do this again.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>
CC: Jan Beulich <jbeulich@suse.com>
Reported-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

Config.mk

@@ -237,9 +237,7 @@ ifneq (,$(CONFIG_QEMU))
 QEMU_TRADITIONAL_LOC ?= $(CONFIG_QEMU)
 endif
 ifneq (,$(QEMU_TAG))
-QEMU_TRADITIONAL_REVISION ?= ab42b4408cb4fc4f869d73218e3d2034e6f5e8ac
-# Tue Mar 31 16:27:45 2015 +0100
-# xen: limit guest control of PCI command register
+QEMU_TRADITIONAL_REVISION ?= $(QEMU_TAG)
 endif
 
 ifeq ($(GIT_HTTP),y)