libvirt: remove flawed get_num_instances method impl

The get_num_instances method impl simply asks libvirt
how many running domains there are. This is flawed for
a number of reasons.

 - It doesn't filter out 'Domain-0' on Xen
 - It doesn't count inactive instances

As a result the value is returns is inconsistent with
the number of instances shown by 'list_instances'.
This will cause the _sync_power_states method in the
compute manager to constantly try to re-sync the list
of instances.

Remove the flawed impl and just allow the base class
default impl to be used. This just does a len() call
over the return of 'list_instances'. Previously this
was quite a heavy API call, but with new enough libvirt
it can be completed in a single API call which should
be fine.

Blueprint: libvirt-domain-listing-speedup
Change-Id: I96242717afa5c928f1cfb5f307b71739f2147e69
Re-add H803 to flake8 ignore list.

I6c2537dd27c947e36ebf37eb3b5c8a1ab8b026a1 mistakenly removed H803 since
running flake8 had no hits for this test, but H803 is the period in the
commit message which we are explicitly ignoring until the next version
of hacking is released (which will remove this check).

Change-Id: I22860cc61336ef5bf8ce4509fe130160bdf7e45b
Merge "libvirt: remove flawed get_num_instances method impl"
Merge "Re-add H803 to flake8 ignore list."