For a long time we've masked out VIR_DOMAIN_BLOCK_COPY_SHALLOW if
there's no backing chain for the copied disk to simplify the code.

One of the refactors of the block copy code caused that we no longer
update the 'flags' variable just the local copies. This was okay until
in ccd4228a we started storing the job flags in the block job data.

Given that we modify how we call qemu we also should modify @flags so
that the correct value is recorded in the block job data.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>