Add 'HvInstanceState' which represents the state of an instance
(either 'running' or 'shutdown') and fix the remaining hypervisor
backends to return a value of this type.  Before this patch, each
backend was returning their own value, some were returning strings,
others integers.  However, the value that represents the state of an
instance was never actually used in Ganeti, until instance shutdown
detection became necessary.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>