Support building of out-of-tree merged qemu from git

Support building of out-of-tree merged qemu from git

This provides support for building the new merged qemu from
qemu-xen.git, or from a local checkout.

The default is still to use the in-tree version, because the patchman
infrastructure for testing, and propagation from staging to
xen-unstable, needs a small amount of work to cope with git.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>