QMP/input-send-event: make console parameter optional

The 'QemuConsole' is the input source for handler, we share some
input handlers to process the input events from different QemuConsole.

Normally we only have one set of keyboard, mouse, usbtablet, etc.
The devices have different mask, it's fine to just checking mask to
insure that the handler has the ability to process the event.

I saw we try to bind console to handler in usb/dev-hid.c, but display
always isn't available at that time.

If we have multiseat setup (as Gerd said), we only have 'problem' in
this case. Actually event from different devices have the same effect
for system, it's fine to always use the first available handler
without caring about the console.

For send-key command, we just pass a NULL for console parameter in
calling qemu_input_event_send_key(NULL, ..), but 'input-send-event'
needs to care more devices.

Conclusion:
Generally assigning the special console is meanless, and we can't
directly remove the QMP parameter for compatibility.

So we can make the parameter optional. The parameter might be useful
for some special condition: we have multiple devices without binding
console and they all have the ability(mask) to process events, and
we don't want to use the first one.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
