input: Add trace event for empty keyboard queue

When driving QEMU from the outside, we have basically no chance to
determine how quickly the guest OS picks up key events, so we usually
have to limit ourselves to very slow keyboard presses to make sure
the guest always has enough chance to pick them up.

This patch adds a trace events when the keyboarde queue is drained.
An external driver can use that as hint that new keys can be pressed.

Signed-off-by: Alexander Graf <agraf@suse.de>
Message-id: 1490883775-94658-1-git-send-email-agraf@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
