Virt: remove unnecesary return code

The methods attach_volume, detach_volume and swap_volume should
not return anything.

The purpose of the patch set is to have a consistent behaviour across
drivers for the aforementioned methods.

Change-Id: I381ede44d71cd5f6ce98804853e4e8f587b46c99
Make resize raise exception when no valid host found

When resize operation starts, it will ask conductor to find a
valid host and try to resize to that host. When there is no valid
host can be found ,current code return directly without error
notification, which will make upper layer code (e.g novaclient)
thinks the execution is succesfully performed and report 'Finished'.
But actually it isn't due to no valid host was found.

Change-Id: I3d66dbfc0a7346762b920e802aa3be92c43d93b7
Closes-Bug: #1297052
Not count disabled compute node for statistics

No server will be scheduled to disabled compute service, thus we
should not count the corresponding compute node information.

It's arguable if we should count for 'down' service, since service
may be marked down because of communication error. If we do want to
exclude the down service, we need passing the information from caller
because the up/down state is not kepts in database, and it means compute
and cell api changes.

Closes-Bug: #1285259

Change-Id: I5e3e71ef30683c5eb5cc4462f58fa5f29d7c3f4b
Merge "Virt: remove unnecesary return code"
Merge "Make resize raise exception when no valid host found"
Merge "Not count disabled compute node for statistics"