blockjob: relabel entire existing chain

blockjob: relabel entire existing chain

When using block copy to pivot over to a new chain, the backing files
for the new chain might still need labeling (particularly if the user
passes --reuse-ext with a relative backing file name).  Relabeling a
file that is already labeled won't hurt, so this just labels the entire
chain at the point of the pivot.  Doing the relabel of the chain uses
the fact that we already safely probed the file type of an external
file at the start of the block copy.

* src/qemu/qemu_driver.c (qemuDomainBlockPivot): Relabel chain before
asking qemu to pivot.