qemu: snapshot: Deactivate block nodes on manually snapshotted disks

qemu: snapshot: Deactivate block nodes on manually snapshotted disks

If the user wants to manually preserve state of the disk we need, apart
from pausing the machine to quiesce all writes, also deactivate the
block nodes of the device. This ensures that qemu writes out metadata
(e.g. block dirty bitmaps) which are normally stored only in memory,
thus allowing a consistent snapshot including the metadata.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>