CA-83837: lookup a domid from a VM uuid via xenstore rather than the domain info list

Unfortunately some libraries (such as libxenguest) will destroy a domain
beneath us, causing us to lose track of the domid corresponding to a uuid.
We are in control of xenstore, so we use the /vm/uuid/domains map to find the
lookup the domid. Note this also means that 'add' has to add such a key for
already-running VMs (such as dom 0)

Without this change a VM.suspend will leave VDIs attached.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>