Rename Image.copy() to Image.copyCollapsed().

Renaming this method in order to clarify it's result and
differentiate it from dict.copy()

Change-Id: I4b6ec44430465097baf4dd5a2e5bdf6e3842a33f
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18232
Reviewed-by: Dan Kenigsberg <danken@redhat.com>