libqos/ahci: Add ahci_port_check_error helper

ahci_port_check_error checks a given port's error registers and asserts
that everything from the port-level view is still OK.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1423158090-25580-6-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
