This bug is quite similar to bug 1776949, but instead of getting the "unable to find any master var store for loader" message at install time, we get it at live migration time. The same workaround from bug 1776949 comment 1 works to fix migration, even though it's no longer necessary to fix install.
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 (virt:8.3 bug fix and enhancement update), 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-2020:5137