mktarball: For qemu upstream, use their scripts/archive-source.sh

mktarball: For qemu upstream, use their scripts/archive-source.sh

qemu upstream uses git submodules.  git archive does not work with git
submodules (and could not work properly with them, because this is one
of the many things it is inherently impossible to do correctly with
git submodules).

qemu upstream have worked around this by providing a rather scary
shell script which attempts to do roughly the right thing.  It's close
enough that we can use it with only minor precautions.

Unfortunately this does mean that `mktarball' now executes the qemu
source code it was using, rather than merely shuffling it about, as it
did previously.  I think this is a less bad ill than copying (and,
effectively, forking) the scary script.

CC: Wei Liu <wei.liu2@citrix.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>
CC: Juergen Gross <jgross@suse.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>