uhci: Don't allow the guest to set port-enabled when there is no dev connected

It is possible for device disconnect and the guest trying to reset the port
(because of USB xact errors prior to the disconnect getting signaled) to race,
when we hit this race, the guest will write the port-control register with its
pre-disconnect value + the reset bit set, after which we have a disconnected
device with its port-enabled bit set in its port-control register, which
is no good :)

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
