block: add .bdrv_reopen_prepare() stub for iscsi

To suppport reopen(), the .bdrv_reopen_prepare() stub must exist.
iSCSI does not have anything that needs to be done to support reopen,
so we can just implement the _prepare() stub.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>