hv_*: Always return from Verify, style fixes

Change all “Verify” methods in hypervisor abstractions to explicitely
return None if no problem was detected. Remove punctuation from error
messages. Update docstrings with “@return” and some small mistakes.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>