lvcreate/lvconvert: prohibit creation of/conversion to mirrored mirror logs

lvcreate/lvconvert: prohibit creation of/conversion to mirrored mirror logs

In RHEL7 we marked mirrored mirror logs as deprecated and
added a related message.  This patch prohibits creating new
'mirror' LVs with that log type or converting existing LVs
to have one.

Existing LVs with mirrored mirror log can be activated
and converted to disk/core logs.

Avoid double deprecation message when running lvconvert.

Resolves: rhbz1643562