disk: use disk_type argument when creating a disk

this does not seem to be necessary for creating a regular disk
but will be used for snapshot creation

Signed-off-by: William Dauchy <william@gandi.net>
disk: enhance help for disk creation

name is autogenerated if not provided
this make the help more accurate compared to vm creation

Signed-off-by: William Dauchy <william@gandi.net>
disk: add snapshot command

this permits to create snapshots on the fly

Signed-off-by: William Dauchy <william@gandi.net>