common/admin_socket: pass ostream to call() for error output

common/admin_socket: pass ostream to call() for error output

This separates the error stream from the output stream for the synchronous
hook.

This patch includes misc cleanup in the various implementations to make
use of the new stream for errors.

Add a test to unittest_context to ensure we're getting the error stream.

Signed-off-by: Sage Weil <sage@redhat.com>