snapshot: require existence before returning success

snapshot: require existence before returning success

Blindly returning success is misleading if the object no longer
exists; it is a bit better to check for existence up front before
returning information about that object.  This pattern matches the
fact that most of our other APIs check for existence as a side
effect prior to getting at the real piece of information being
queried.

* src/esx/esx_driver.c (esxDomainIsUpdated, esxDomainIsPersistent):
Add existence checks.
* src/vbox/vbox_tmpl.c (vboxDomainIsPersistent)
(vboxDomainIsUpdated): Likewise.