qemu-xen: build adjustments to support out-of-tree builds

qemu-xen: build adjustments to support out-of-tree builds

QEMU by itself can be built outside of its source directory. With the
qemu repository being separate from the hypervisor/tools one it seems
to make sense to make use of this feature, but doing so requires a
couple of adjustments to the Xen changes to it. Basically, if
CONFIG_QEMU is found to indicate an existing directory, this directory
will be used rather than cloning the git repo into the build tree.

[ This changeset is the xen-unstable part of the patch but also
  includes the QEMU_TAG update to pull in the qemu part. -iwj ]

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>