Remove unnecessary wrapper for 5 compute APIs

There are 5 APIs in compute layer:
check_instance_shared_storage
change_instance_metadata
add_fixed_ip_to_instance
remove_fixed_ip_from_instance
reset_network
has unnecessary wrapper, this patch removed them

reverts_task_state is a wrapper to restore instance task state
when some thing wrong in executing the task,
change_instance_metadata, add_fixed_ip_to_instance
remove_fixed_ip_from_instance, reset_network doesn't change the state,
so no need to wrap it

wrap_instance_fault is a wrapper to catch the exception related to
instances, check_instance_shared_storage is not related to it

Change-Id: I2c4779c01854c2c8bac40e7379ad2fc4d0a7478a
objects: restore some datetime field comments

The datetime coercion code in nova.objects.utils had a couple of
helpful comments which were lost when the code was refactored into
nova.objects.fields.

Change-Id: If23320dd8780a1947ad4d5946c55e1e8794b782f
Remove use of service_* conductor calls from xenapi host.py

This removes use of some service calls in the conductor by xenapi's
host.py module and replaces them with object usage. Existing tests
are sufficient to confirm the same behavior remains.

Related to blueprint virt-objects-juno

Change-Id: I908cca4fa1d297c5c9b1d84ae7ebe7452a30ba12
Add create() method to InstanceFault object

This allows creating InstanceFaults through the object interface.

Related to blueprint virt-objects-juno

Change-Id: Iedde2e220127778fa6252d310f9e73c8506f605e
Merge "objects: restore some datetime field comments"
Remove conductor usage from add_instance_fault_from_exc()

This makes add_instance_fault_from_exc() use the InstanceFault
object instead of a raw conductor call. This is actually a
minor change except that all callers of this method must now
stop passing the conductor API, and tests must return something
valid from the mocked call.

Related to blueprint virt-objects-juno

Change-Id: I49b17c1ef3d14fd36f016c37d88287e62126bf68
Merge "Remove unnecessary wrapper for 5 compute APIs"
Merge "Remove use of service_* conductor calls from xenapi host.py"
Merge "Add create() method to InstanceFault object"
Merge "Remove conductor usage from add_instance_fault_from_exc()"