Use Log instead of Logs

[palainp]

I don't know if this was intentional but the logs print currently doesn't seems to be homogeneous, using Logs:

chamelon/mirage/fs.ml

Line 89 in 2481dfd

Logs.err (fun m -> m "error reading block pair %Ld, %Ld (0x%Lx, 0x%Lx): %a"

will print a line with label [application] while with Log:

chamelon/mirage/fs.ml

Line 50 in 2481dfd

Log.err (fun f -> f "error reading block %Ld : %s"

the label will be clearer ([chamelon-fs] or equivalent for other .ml).

This PR changes Logs into Log to have each line associated with the correct log label.

[yomimono]

It absolutely wasn't intentional! I appreciate the fix.