virsh-domain: Fix return of virshGetDBusDisplay() in one error path

virsh-domain: Fix return of virshGetDBusDisplay() in one error path

The virshGetDBusDisplay() function is declared to return a
pointer and yet, in one error path false is returned. Switch the
statement to return NULL, which is what other error paths use to
indicate an error.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>