hw/i386: fix vmmouse registration

hw/i386: fix vmmouse registration

According to the logic of vmmouse_update_handler function,
vmmouse should be registered as an event handler when
it's status is zero.
vmmouse_read_id resets the status but does not register
the handler.
This patch adds vmmouse registration and activation when
status is reset.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Message-Id: <163524204515.1914131.16465061981774791228.stgit@pasha-ThinkPad-X280>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>