travis: reduce time taken for trace-backend testing

travis: reduce time taken for trace-backend testing

These builds are reaching regular timeouts and probably don't need to
be so widely exercised. ftrace and ust in particular are used in
conjunction with whole system profiling which makes most sense with
KVM setups, hence the native softmmu target.

We also expand simple to cover the multiple log backends while
restricting its scope to user-mode testing only.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>