librbd: fixed garbage output from test LibRBD.TestIOPP

buffer::list::c_str does not actually provide a C-style, NULL
terminated string.  As a result, its use for console output
resulted in stray garbage characters being printed.

Fixes: #9405
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #3182 from ceph/wip-9405

librbd: fixed garbage output from test LibRBD.TestIOPP

Reviewed-by: Loic Dachary <ldachary@redhat.com>