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Description of problem:
Referring to members of the backing chain via filename is finicky ( https://bugzilla.redhat.com/show_bug.cgi?id=1614740 ) and requires 
digging through the backing chain data returned by query-block to find the name that qemu will accept ( https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/qemu/qemu_monitor_json.c;h=3181805f9ce79d30b9aa011e9c5859576638f52f;hb=HEAD#l4474 ).

With -blockdev we will specify node names which are meant to refer to backing chain members but block-commit does not support them as arguments.

Upstream posting:
https://lists.gnu.org/archive/html/qemu-block/2018-08/msg00331.html

Requesting blocker+ because this is a missing piece for blockdev integration, which we need in RHEL-7.6 as it's critical to layered products.

Fix included in qemu-kvm-rhev-2.12.0-16.el7

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:3443