We are about to add 6 new values to fetch. This will put us over the
current limit of 16 (we're at 13 now).

Once there are more than 16 parameters, this will affect existing clients
that attempt to fetch blockiotune config values for the domain from the
remote host since the server side has no mechanism to determine whether
the capability for the emulator exists and thus would attempt to return
all known values from the persistentDef. If attempting to fetch the
blockiotune values from a running domain, the code will check the emulator
capabilities and set maxparams (in qemuDomainGetBlockIoTune) appropriately.

On the client side of the remote connection, it uses this constant in
xdr_remote_domain_get_block_io_tune_ret and virTypedParamsDeserialize
calls, so if a remote server returns more than 16 parameters, then the
client will fail with "Unable to decode message payload".
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>