xen: improvement to wait_for_devices()

When printing a warning about a timed-out device, print the
current state of both ends of the device connection (i.e., backend as
well as frontend).  This backports half of changeset 146 from the
Xenbits tree.

Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>