shut down domain when last vCPU goes down

shut down domain when last vCPU goes down

I've just had to deal with an early boot crash of Linux which occurred
so early that even "earlyprintk=xen" did not produce any useful output.
Hence the domain appeared to hang, while in fact it had brought down its
only vCPU. By translating this to a shutdown, the situation will be
better recognizable.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>