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

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>