When using blockdev configurations the 'device' argument of
'blockdev-mirror' must correspond to the topmost node in the block node
graph. Libvirt didn't do this properly in case when 'copy_on_read'
option was enabled on the disk.

Use qemuDomainDiskGetTopNodename to fix it for the blockdev-mirror calls
in qemuDomainBlockCopy and the non-shared-storage migration.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>