vfio-pci: Fix BAR->VFIODevice translation in

vfio-pci: Fix BAR->VFIODevice translation in

DO_UPCAST is supposed to translate from the first member of a struct to
that struct, not from arbitrary ones. And it (usually) breaks the build
when neglecting this rule. Use container_of to fix the build breakage
and likely also the runtime behavior.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
aw: runtime behavior is actually the same, but clearly misuse of DO_UPCAST
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>