vfio: Start improving VFIO/EEH interface

At present the code handling IBM's Enhanced Error Handling (EEH) interface
on VFIO devices operates by bypassing the usual VFIO logic with
vfio_container_ioctl().  That's a poorly designed interface with unclear
semantics about exactly what can be operated on.

In particular it operates on a single vfio container internally (hence the
name), but takes an address space and group id, from which it deduces the
container in a rather roundabout way.  groupids are something that code
outside vfio shouldn't even be aware of.

This patch creates new interfaces for EEH operations.  Internally we
have vfio_eeh_container_op() which takes a VFIOContainer object
directly.  For external use we have vfio_eeh_as_ok() which determines
if an AddressSpace is usable for EEH (at present this means it has a
single container with exactly one group attached), and vfio_eeh_as_op()
which will perform an operation on an AddressSpace in the unambiguous case,
and otherwise returns an error.

This interface still isn't great, but it's enough of an improvement to
allow a number of cleanups in other places.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
