VMware: Improve logging on failure due to invalid guestId

If a glance image has a vmware_ostype property which doesn't
correspond to a valid VMware guestId, VM creation will fail with the
unhelpfully generic error message:

A specified parameter was not correct.

This change improves this situation slightly by logging a warning if
VM creation fails and guestId is not a recognised value. We limit
ourselves to logging a warning because we don't want to tightly couple
ourselves to the vSphere behaviour. We can't guarantee that this is
the reason the RPC call failed, or that it is the only reason the RPC
call failed.

DocImpact

This change doesn't impact the behaviour of the VMware driver except
that it will emit an additional warning message to the logs under the
circumstances described. However, given the potential for
user misconfiguration, we should describe this pitfall in the docs.

Closes-Bug: #1357263
Change-Id: I10e0982c6c450facdeea41c7f1d266e20a924d08
Merge "VMware: Improve logging on failure due to invalid guestId"