implement suspend to disk for running vms

implement suspend to disk for running vms

the idea is to have the same logic as with snapshots, but without
the snapshotting of the disks, and after saving the vm state (incl memory),
we hard shut off the guest.

this way the disks will not be touched anymore by the guest

to prevent any alteration of the vm (incl migration, hw changes, etc) we
add a config lock 'suspend'

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>