iotests: record separate timings per format,protocol pair

The 'check' program records timings for each test that
is run. These timings are only valid, however, for a
particular format/protocol combination. So if frequently
running 'check' with a variety of different formats or
protocols, the times printed can be very misleading.

Instead of having a single 'check.time' file, maintain
multiple 'check.time-$IMGPROTO-$IMGFMT' files.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170103160556.9895-1-berrange@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
