qemu: make processNetDevStreamDisconnectedEvent() reusable

qemu: make processNetDevStreamDisconnectedEvent() reusable

We will be adding a new event whose response will be *exactly* the
same as the response to NETDEV_STREAM_DISCONNECTED. Rather than doing
a copy-paste of the complete function that does the processing, turn
that function into something more generic that takes the name of the
event as an arg (the event name is only used in log messages).

Signed-off-by: Laine Stump <laine@redhat.com>
Tested-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>