Added supports_snapshots? method to base/sub class.

Added supports_snapshots? method to base  class that returns false and added supports_snapshots? to VmVmware subclass to return true as that allows snapshot functionality for VM .

https://bugzilla.redhat.com/show_bug.cgi?id=1141174
https://bugzilla.redhat.com/show_bug.cgi?id=1141375