spapr: Cleanups relating to DRC awaiting_release field

'awaiting_release' indicates that the host has requested an unplug of the
device attached to the DRC, but the guest has not (yet) put the device
into a state where it is safe to complete removal.

1. Rename it to 'unplug_requested' which to me at least is clearer

2. Remove the ->release_pending() method used to check this from outside
spapr_drc.c.  The method only plausibly has one implementation, so use
a plain function (spapr_drc_unplug_requested()) instead.

3. Remove it from the migration stream.  Attempting to migrate mid-unplug
is broken not just for spapr - in general management has no good way to
determine if the device should be present on the destination or not.  So,
until that's fixed, there's no point adding extra things to the stream.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Daniel Barboza <danielhb@linux.vnet.ibm.com>