Set errp to report errors up to the right monitor.

Use error_append_hint() to give hints about parameters on !qmp monitors,
instead of a direct fprintf() call.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>