When using blockdev configurations the 'device' argument of
'blockdev-commit' 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 when calling block-commit.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>