build: don't fail if given a git submodule which does not exist

If going back in time in git history, across a commit that introduces a new
submodule, the 'git-submodule.sh' script will fail, causing rebuild to fail.

This is because config-host.mak contains a GIT_SUBMODULES variable that lists
a submodule that only exists in the later commit. config-host.mak won't get
repopulated until config.status is invoked, but make won't get this far due to
the submodule error.

This change makes 'git-submodule.sh' check whether each module is known to git
and drops any which are not present. A warning message will be printed when any
submodule is dropped in this manner.

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