VMware: improve log message for attachment of CDROM

Commit 3c59d998c5e6b80549053b01f919ac318cd31b13 config drive
support. The log message makes use of the instance_name which is
not unique. The may be confusing when debugging.

TrivialFix

Change-Id: Iff7f4f938f4980f0c0bf2dc3017ed741453a69f5
Stop depending on sitepackages libvirt-python

We can install libvirt-python bindings from pypi now because the libvirt
devs are supporting pypi packages and the version of libvirt we are
testing nova master against is new enough to link against that pypi
package.

Making this switch will better isolate nova test environments as all
dependencies can be installed without any site packages leaking into
virtualenvs. It will also make the dependency on libvirt-python a bit
more explicit.

Change-Id: I2890702869f05a02ad5e2ecb419db06433231b36
Use library instead of CLI to cleanup RBD volumes

'rbd list' CLI returns error code when there are no rbd volumes, which
causes problems during live migration of VMs with RBD backed ephemeral
volumes. It's safer to use the library that only raises an exception in
case of a real problem.

The only case where rbd CLI is still justified is import, which is
needed to correctly import sparse image files.

All code related to cleanup of RBD volumes is moved to rbd.py, this
fixes a yo-yo problem with single-use methods scattered across 3
different files, and minimizes impact of this fix on imports in
imagebackend and utils.

Closes-Bug: #1346092
Change-Id: I92cd6b16fbd93b377fe47b15d22efbbf68d02513
Signed-off-by: Dmitry Borodaenko <dborodaenko@mirantis.com>
Merge "Use library instead of CLI to cleanup RBD volumes"
Merge "Stop depending on sitepackages libvirt-python"
Merge "VMware: improve log message for attachment of CDROM"