qemu: Don't reuse variable in processSerialChangedEvent()

qemu: Don't reuse variable in processSerialChangedEvent()

When a VSERPORT_CHANGE event is processed, we firstly do a little
detour and try to detect whether the event is coming from guest
agent. If so, we notify threads that are currently talking to the
agent about this fact. Then we proceed with usual event
processing (BeginJob(), update domain def, emit event, and so
on).

In both cases we use the same @dev variable to refer to domain
device. While this works, it will make writing semantic patch
unnecessary harder (see next commit(s)). Therefore, introduce a
separate variable for the detour code.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Claudio Fontana <cfontana@suse.de>