[Libguestfs] [PATCH nbdkit 0/7] server: Allow datapath debug messages to be suppressed.

The immediate reason for this patch is to reduce the amount of debugging in virt-v2v with using the virt-v2v -v option (because this implies running nbdkit in verbose mode too). Most of the messages are datapath ones about pread/pwrite requests, and in fact as we've added more filters on top of nbdkit these messages have got more and more verbose. However they are not particularly interesting so by using -D nbdkit.backend.datapath=0 we could suppress all these messages, reducing the size of the log files by over 2/3rds. Rich.