[CONSOLED] Shouldn't call domain_create_ring() for a domU which is shutting down.

[CONSOLED] Shouldn't call domain_create_ring() for a domU which is shutting down.
There is a race condition between domain_create_ring() and the removal
of the domU entry from xenstore. If the removal of the domU entry from xenstore
comes later than the first xs_gather() call in domain_create_ring(), it will go
on and bind the evtchn port to the dying domU, which will cause below error
message: (XEN) (file=event_channel.c, line=144) EVTCHNOP failure: domain 0, error -22, line 144

Signed-off-by: Max Zhen <max.zhen@sun.com>