Switching to merged (git) qemu by default

Switching to merged (git) qemu by default

The merged qemu tree is now on xenbits at:
   http://xenbits.xensource.com/git-http/qemu-xen-unstable.git
(and a corresponding staging tree in /staging).

That's a raw git tree fetchable by `git-clone http://....', so not
really browseable.  There will be gitweb, and a real git server, set
up shortly, as well as a one-way Mercurial gateway.

The tools/ioemu subdirectory will remain in the xen-unstable tree for
this release, as an alternative.

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