Per the qapi schema, block_set_io_throttle takes most arguments
as ints, not strings.

* qmp-commands.hx (block_set_io_throttle): Use correct type.  Fix
whitespace and a copy-paste bug in the process.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>