qapi: add block latency histogram interface

qapi: add block latency histogram interface

Set (and clear) histograms through new command
block-latency-histogram-set and show new statistics in
query-blockstats results.

For now, the command is marked experimental with prefix 'x-',
to gain experience with the interface without being stuck
with design decisions.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180309165212.97144-3-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[eblake: fix typos, mention x- prefix in commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>