block: Add notes to iSCSI's .bdrv_open and .bdrv_reopen_prepare

iSCSI currently does not need to do any actions to support the
current usage of bdrv_reopen().  However, it is important to note
a couple of things: 1.) A connection will not be re-established to
an iSCSI target, and 2.) If iscsi_open() is changed to parse 'flags',
then iscsi_reopen_prepare() may need to be more than a stub.

In light of the above, this commit adds comments above both of the
functions to bring attention to these facts.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>