esx: Fetch snapshot info directly for filtering

esx: Fetch snapshot info directly for filtering

When fetching domains with virConnectListAllDomains() and when filtering
by snapshot existence is requested the ESX driver first lists all the
domains and then check one-by-one for snapshot existence. This process
takes unnecessarily long time.

To significantly improve the time necessary to finish the query we can
request the snapshot related info directly when querying the list of
domains from VMware.

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>