CA-135017: notify toolstack about tapdisk unpause errors in coalesce

Coalesce is an asynchronous operation so if a tapdisk refresh fails the
VM will lose the disk but the administrator will realise that when the
VM hangs. This patch notifies the toolstack as soon as the failure
occurs.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Acked-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #147 on xapi-project/sm