vdsClient: check getFileList arguments

A simple check of getFileList arguments on the function call.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=958878
Signed-off-by: Peter V. Saveliev <peet@redhat.com>
Change-Id: I0519562fd459393a72644b5d4e2045cb77b22746
Reviewed-on: http://gerrit.ovirt.org/16380
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>