Add DTS_MIRRORED frozenset

Use DTS_MIRRORED to indicate mirrored disk templates that allow
migrations/failover.

DTS_MIRRORED is the union of DTS_EXT_MIRROR and DTS_NET_MIRROR.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>