qtest/ahci: Assert sector size in identify test

A minor sanity check to assert that the sector size is 512.
The current block layer code deeply assumes that the IDE
sector size will be 512 bytes, so we carry forward that assumption
here.

This is useful for the DMA tests, which currently assume that
a sector will always be 512 bytes.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1423158090-25580-19-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
