Disallow the creation of mirrors (mirror or raid1 segtype) with only one leg.

Disallow the creation of mirrors (mirror or raid1 segtype) with only one leg.

If you specify the segment type (e.g. --type mirror) and the mirrors argument
as zero, it would result in a mirrored LV with only one image.  While the device
may be valid in theory, it should not be allowed in practice.  It also makes it
difficult on the conversion tools, since they react badly to single-image
mirrors.