renamed `getdisksize` to `get_disksize`
helper functions to for working with disks

virsh_getdisks (): list all disk devices of a virtual host
lvm_filterdisks(): extract all LV-devices from a list of block-devices
get_snapname   (): calculate full snapshot-device-name from the
                   snaphost name and the full snapped device name.
added basic automatic-mode

if no VDISKS are specified, this automatically uses the disks of the vhost

TODO: filter out all non-LVM devices (even if they were given manually!)
filter LVM-disks from given VDISKS
no more SNAPBASE fuddling...
verbose/debug functions
fixing usage messages
adding loads of 'debug' foo
setting debug-level via cmdline
better usage message (for verbosity)
'output' function for debugging
debugPipe() helper for pipe-debugging

slightly different debug-output than `debug()`
using debugPipe in pipes