KVM: Increase NR_IOBUS_DEVS limit to 200

KVM: Increase NR_IOBUS_DEVS limit to 200

(Cherry-picked from commit e80e2a60ff7914dae691345a976c80bbbff3ec74)

This patch increases the current hardcoded limit of NR_IOBUS_DEVS
from 6 to 200. We are hitting this limit when creating a guest with more
than 1 virtio-net device using vhost-net backend. Each virtio-net
device requires 2 such devices to service notifications from rx/tx queues.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>