qemu: snapshot: Don't mark all block disks for metadata reuse

qemu: snapshot: Don't mark all block disks for metadata reuse

For block devices used as snapshot source the new snapshot code would
set the reuse flag. This inhibits to take snapshot without specially
preparing the block image before taking the snapshot.

Fortunately this is not a regression as only the new way of specifying
snapshot source is affected.

For the followin snapshot XML:
 <domainsnapshot>
   <disks>
     <disk name='vda' type='block'>
       <driver type='qcow2'/>
       <source dev="/dev/andariel/testsnap" />
     </disk>
   </disks>
 </domainsnapshot>

You'd get:
error: internal error: unable to execute QEMU command 'transaction': Image is not in qcow2 format

After this patch the snapshot is created successfully.