guest-os: Ubuntu.cfg use perf profiler as default

kvm_stat is not packaged for Ubuntu, please use perf by default.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
guest-os: Ubuntu.cfg change cpu_chk_cmd

The old style of command causes issues because of the $ character in
Ubuntu environments causing tests to fail. Replace with a safer command.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>