nvmecontrol: Appease a meaningless signed error warning thing

Consistently use the weird ssize_t type for things I know are unsigned,
but which none-the-less need to be compared to signed things to prevent
a bogus warning. Sigh, these aren't bugs, can't possibly be bugs and
a waste of time to fix.

Sponsored by: Netflix

nvmecontrol: Implement telemetry-log command.

This produces the same data as the Linux nvme-cli 'nvme telemetry-log'
command. It extracts the telemetry log from drive. This is a variable
length log, so we read the first page and find out how much of the log
to grab. There's 3 levels of details available, and we grab the level of
detail specified on the command line.

Sponsored by: Netflix