I can reproduce it with virt-viewer-2.0-7.el7.x86_64,
then updated to virt-viewer-2.0-8.el7.x86_64 and tried it again, it doesn't occur any more.
steps:
1. Set monitor-mapping to blank string
$ cat .config/virt-viewer/settings
[4987d17c-4c07-432a-918d-47e1a1ad79fc]
monitor-mapping=
2. Wait for the guest startup and Login into the guest first.
3. lanuch virt-viewer with -f arguments.
$ virt-viewer -c qemu:///system rhel7.2-back-clone -f
Actual Result:
virt-viewer can open guest successfully and there is a warning message in terminal.
(virt-viewer:908): virt-viewer-WARNING **: Empty monitor-mapping configuration
so move this bug from ON_QA to VERIFIED.
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://rhn.redhat.com/errata/RHBA-2016-2229.html