vfio: free dynamically-allocated data in instance_finalize

In order to enable out-of-BQL address space lookup, destruction of
devices needs to be split in two phases.

Unrealize is the first phase; once it complete no new accesses will
be started, but there may still be pending memory accesses can still
be completed.

The second part is freeing the device, which only happens once all memory
accesses are complete.  At this point the reference count has dropped to
zero, an RCU grace period must have completed (because the RCU-protected
FlatViews hold a reference to the device via memory_region_ref).  This is
when instance_finalize is called.

Freeing data belongs in an instance_finalize callback, because the
dynamically allocated memory can still be used after unrealize by the
pending memory accesses.

This starts the process by creating an instance_finalize callback and
freeing most of the dynamically-allocated data in instance_finalize.
Because instance_finalize is also called on error paths or also when
the device is actually not realized, the common code needs some changes
to be ready for this.  The error path in vfio_initfn can be simplified too.

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