pci: split shpc_cleanup and shpc_free

object_unparent should not be called until the parent device is going to be
destroyed.  Only remove the capability and do memory_region_del_subregion
at unrealize time.  Freeing the data structures is left in shpc_free, to
be called from the instance_finalize callback.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
