usb-mtp: add support for basic mtp events

When the host polls for events, we check our
events qlist and send one event at a time. Also, note
that the event packet needs to be sent in one go, so
I increased the max packet size to 64.

Tested with a linux guest.

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-id: 1448314625-3855-5-git-send-email-bsd@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
