virttest.qemu_devices: Lose verbose=False incompatible with at 0.14

This is an attempt to better fix the issue where system_output
doesn't have the verbose parameter in autotest 0.14. It is better
and less desruptive to simply lose the parameter, and we can
get it back once 0.15 rpms are available on Fedora.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>