Refactor: libcib: Check doc in cib_file_perform_op_delegate()

For safety, make a copy of output if it's in the same XML doc as the
in-memory CIB, instead of only making a copy if they both point to the
same location. The hope is to reduce the likelihood of tricky libxml2
bugs.

Ref T185

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>