Remove dead variable assignment in compute manager

The assigement of 'db_power_state' in '_sync_power_state' is
never used before it is overwritten a few lines later

Blueprint: compute-driver-events
Change-Id: I0a7e88fc7c3067a09b0a05a643ae4e6d40a11488
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Split out body of loop in _sync_power_states in compute manager

The _sync_power_states method in the compute manager iterates
over all instances in the DB, synchronizing their power
states. The introduction of async events to the compute
drivers requires a method for synchronizing the power state
of a single instance at a time. Split out the body of the
loop in _sync_power_states to a method _sync_instance_power_state

Blueprint: compute-driver-events
Change-Id: I5eee72b39f765a2444388005fabba2cff7ff4b29
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Remove dead variable assignment in compute manager"
Merge "Split out body of loop in _sync_power_states in compute manager"