Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

GEOM virstor .so does not need libmd.

Rename the generic "CLASS" to the more specific "GEOM_CLASS".
While I'm here remove redundancy and inconsistencies.

Obtained from: Juniper Networks

Bring in the GEOM Virtualisation class, which allows to create huge GEOM
providers with limited physical storage and add physical storage as
needed.

Submitted by: Ivan Voras
Sponsored by: Google Summer of Code 2006
Approved by: re (kensmith)