Port xen-api f81f53eb5a55bd6a1d605926aeddd5fd50fdd78f

Plumb through xenstore, console backend domids
Update following xenctrl interface changes in xen-4.2
If we're in dom0 then 'xenstore-read domid' will probably fail, since no-one will have created the key for us.

In this special case, we assume we're in domid 0. We assume all other domains
will have been created with a valid 'domid' key.
Xenctrl.domain_set_memmap_limit can fail: ignore the error
Plumb store_domid and console_domid through some places.
Remove the "sanity check" which causes backend "vanished" exceptions erroneously