Config.mk: commented-out CONFIG_QEMU example now uses pwd/$(XEN_ROOT)

If you actually set it to a relative path, the qemu build breaks.
So this commented-out rune (an example) should arrange to be absolute.

Unfortunately XEN_ROOT is itself relative so the previous attempt to
fix this (22772:654563af359f) didn't work.  So use `pwd`.

Tested-by: M A Young <m.a.young@durham.ac.uk>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

Config.mk

@@ -182,7 +182,7 @@ endif
 
 # Specify which qemu-dm to use. This may be `ioemu' to use the old
 # Mercurial in-tree version, or a local directory, or a git URL.
-# CONFIG_QEMU ?= $(XEN_ROOT)/../qemu-xen.git
+# CONFIG_QEMU ?= `pwd`/$(XEN_ROOT)/../qemu-xen.git
 CONFIG_QEMU ?= $(QEMU_REMOTE)
 
 QEMU_TAG := xen-4.1.0-rc2