make: Check tools/qemu-xen[-traditional] for qemu before downloading

make: Check tools/qemu-xen[-traditional] for qemu before downloading

Currently xen, qemu-xen, and qemu-xen-traditional are kept in separate
repositories, but when we release them as a tarball, qemu-xen and
qemu-xen-traditional are in-lined into the tools/ directory.

In order to make this "just work", at the moment developer doing the
release manually modifies Config.mk as part of the relase process so
that CONFIG_QEMU and QEMU_UPSTREAM_URL point into the tools/ directory
instead.

Modify Config.mk to automatically check there before trying a remote
repository.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>