Move calls to os.path.exists() in libvirt imagebackend

The checks for the path existence were removed from the base Image class
to avoid redundant checks (some of implementations also perform them and
behave differently depending on the results). Also special handling for
some backends were removed from the base Image class.

Change-Id: Ie6cdf0d6d9801f84fae7a58c1034547611712323
Closes-Bug: #1152748
Merge "Move calls to os.path.exists() in libvirt imagebackend"