usersguide: first hint of a userguide

Written only few paragraph. It is still missing a lot of stuffs
commands: support hybrid VM disk image

now virtual machine can have both local file disk image as well
well remote iscsi based disk image.

If the set host vm nas value is not set for a given virtual machine
it will booted using the backing storage specified in its original
configuration (rocks list host vm showdisks=1).
Once the vm nas values is set, the iSCSI solution will be used,
and the original configuration will be ignored except for the size.
commands: add rocks dump to backup nas information

add rocks dump host vm nas to backup and restore values for vm nas