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 is the qemu-xen half. -iwj ]

Signed-off-by: Jan Beulich <jbeulich@novell.com>