xenapi: set vcpu cap to ensure weight is applied

There is a bug in XenServer 6.1 and 6.2 that means the vcpu weight
we set is ignored if the cap is not also set.

Setting the cap to its default of 0, meaning no cap, to ensure the
weight value is applied.

Fixes bug 1213960
Change-Id: I367a9afe188734cf5e95c01fc014b9044003d6e3
Check instance exists or not when evacuate

When evacuate instance to another host, similar to instance create,
nova compute must make sure the instance not yet exist on the host.

Currently there is no check for this, the fix was add the checking
of instance exist or not when evacuate.

Fix bug 1216501

Change-Id: I3ccbd5db5174904e17ffa8f9236ca4e1a09f4dbe
Fix stopping instance in sync_power_states

When an instance is marked as stopped in the DB but is actually running,
the sync_power_states periodic task will try to stop it.  It does this
using the compute API.  The compute API checks the vm state before
proceeding.  The STOPPED state is treated as invalid.  This patch allows
the caller to bypass the state check and makes use of that in this
periodic task.

Change-Id: I447b6dbd1da72ed33067659708e1755bf8c933d0
Closes-bug: #1226965
Merge "Fix stopping instance in sync_power_states"
Merge "Check instance exists or not when evacuate"
Merge "xenapi: set vcpu cap to ensure weight is applied"