Both users of RateLimit, block-copy.c and blockjob.c, treat
a speed of zero as unlimited, while RateLimit treats it as
"as slow as possible".  The latter is nicer from the code
point of view but pretty useless, so disable rate limiting
if a speed of zero is provided.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210614081130.22134-2-eesposit@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>