When RHEL 7.5 comes out (sometime in April-2018), the following Nova
upstream commit should be backported to RHOS-11 (and RHOS-10 — given its 
long-term support cycle):

    commit 807579755c4a116309eca5b2bcdbab9d1f393bab
    Author: Matt Riedemann <mriedem.os>
    Date:   Wed Sep 20 10:44:11 2017 -0400

        Support qemu >= 2.10

        Qemu 2.10 added the requirement of a --force-share flag to qemu-img
        info when reading information about a disk that is in use by a
        guest. We do this a lot in Nova for operations like gathering
        information before live migration.

        [...]

RHEL 7.5 will ship with QEMU 2.10, which will bring in an additional
flag '--force-share' — this is mandatory when running `qemu-img info` on
a live Nova instance.

Nova does this `qemu-img info` call in several places.

This is already bacported to Pike (RHOS-12).  But needs to be backported to
RHOS-11 (and RHOS-10), given their support life-cycles:

    RHOS-11: End of Production, Phase 2: May 18, 2018
    RHOS-10: End of Production, Phase 2: December 16, 2019

    * * *

RHOS-11 seems a bit on the border on the question of whether to this 
backport or not.  As it can be seen, its "End of Production, Phase 2" is 
on 18-May-2018.  And RHEL-7.5 comes out sometime in early April-2018.  
So there's a small window of about a month where RHOS-11 users
upgrading to RHEL 7.5 will see broken Nova.