qemu: block: fix error when blockcopy target is librbd encrypted

qemu: block: fix error when blockcopy target is librbd encrypted

Encryption secrets are considered a format dependency, even
when being used by the storage node itself, as in the case of
using encryption engine=librbd.
Currently, the storage node is created (blockdev-add) before
creating the format dependencies (including encryption secrets).
As a result, when trying to perform a blockcopy when the target
disk uses librbd encryption, an error of this form is returned:

  "error: internal error: unable to execute QEMU command 'blockdev-add': No secret with id 'libvirt-5-format-encryption-secret0'"

To overcome this error, we change the order of commands so that
format dependencies are created BEFORE creating the storage node.

Signed-off-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>