conf: snapshot: Introduce 'manual' mode for snapshot of a disk

conf: snapshot: Introduce 'manual' mode for snapshot of a disk

The idea of the manual mode is to allow a synchronized snapshot in cases
when the storage is outsourced to an unmanaged storage provider which
requires cooperation with snapshotting.

The mode will instruct the hypervisor to pause along when the other
components are snapshotted and the 'manual' disk can be snapshotted
along. This increases latency of the snapshot but allows them in
otherwise impossible situations.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>