build: don't create temporary files in source dir

There are cases where users do VPATH builds with the source directory being on
a read-only volume. In such a case they have to manually run the command
'git-submodule.sh ...modules...' ahead of time. When checking for status we
should not then write into the source dir.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
