VFIOGroup.device_list is effectively our reference tracking mechanism
such that we can teardown a group when all of the device references
are removed.  However, we also use this list from our machine reset
handler for processing resets that affect multiple devices.  Generally
device removals are fully processed (exitfn + finalize) when this
reset handler is invoked, however if the removal is triggered via
another reset handler (piix4_reset->acpi_pcihp_reset) then the device
exitfn may run, but not finalize.  In this case we hit asserts when
we start trying to access PCI helpers since much of the PCI state of
the device is released.  To resolve this, add a pointer to the Object
DeviceState in our common base-device and skip non-realized devices
as we iterate.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>