i18n-key and local-storage hard-coded in xenapi

In order to install/spawn instances, the XenAPI plugin looks for a specific SR on a XenServer/XCP host. More precisely, the plugin assumes the presence of a local SR, as per default XenServer install; this SR has the "other-config" parameter set to "i18n-key=local-storage".

If we look at the code (available in /virt/xenapi/vm_utils.py), this is clearly shown in find_sr:

    @classmethod
    def find_sr(cls, session):
        """Return the storage repository to hold VM images"""
        host = session.get_xenapi_host()

        for sr_ref, sr_rec in cls.get_all_refs_and_recs(session, 'SR'):
            if not ('i18n-key' in sr_rec['other_config'] and
                    sr_rec['other_config']['i18n-key'] == 'local-storage'):
                continue
            for pbd_ref in sr_rec['PBDs']:
                pbd_rec = cls.get_rec(session, 'PBD', pbd_ref)
                if pbd_rec and pbd_rec['host'] == host:
                    return sr_ref
        return None

Many people stumbled upon this, recently and in the past (eg. https://lists.launchpad.net/openstack-xenapi/msg00019.html), so it's time we move this into flags and log a meaningful exception. Plus, in cases where the XenServer only has shared storage, looking for 'local-storage' does feel wrong.