Added sparse zvol support to ZFS.

To enable, add "sparse" to the ZFS storage section, you wish to use it on, in "/etc/pve/storage.pm"

Signed-off-by: Chris Allen <Chris Allen>
Added "nowritecache" option to ZFS storage plugin. Turns off write caching on Comstar and Istgt LUNs created by the plugin. Has no effect on IET, as IET always uses write-through caching for blockio volumes not in read-only mode.

Signed-off-by: Chris Allen <Chris Allen>
Fixed command substitution and output redirection in Istgt module to work in csh - the default shell in FreeBSD. Changed Istgt "restart" to "onerestart". This way a running istgt daemon will always restart whether or not it is enabled in rc.conf.

Signed-off-by: Chris Allen <Chris Allen>
Fixed Istgt LUN Options handling.

Signed-off-by: Chris Allen <Chris Allen>
Added target-group and host-group options for ZFS comstar views. Use "comstar_tg" and "comstar_hg" in "/etc/pve/storage.pm".

Signed-off-by: Chris Allen <Chris Allen>
Added code to abort a snapshot rollback in ZFS if the snapshot selected is not the most recent. This is to avoid lu deletion when trying to rollback to a snapshot that isn't the newest.

Signed-off-by: Chris Allen <Chris Allen>
bump version to 3.0-20