netperf: collect host/guest sysinfo by custom-built  scripts

User wants to collect sysinfo of host/guest, but different users
might have different requests, and many kinds of system might be
used. So I would like to use custom-built scripts to collect the
information that will make users happy.

User needs to add shell scripts to autotest/client/tests/virt/shared/scripts/,
and set the script path in config file.

eg:
 #log_hostinfo_script = scripts/rh_perf_log_hostinfo_script.sh
 #log_guestinfo_script = scripts/rh_perf_log_guestinfo_script.sh
 #log_guestinfo_exec = bash
 #log_guestinfo_path = /tmp/log_guestinfo.sh

Then the script output will be saved to $test.resultsdir/sysinfo,
analyzer can use 'diff' to compare the system information when
regression occurs.

I will send a pull request to add to example scripts to virt-tests repo.

Signed-off-by: Amos Kong <akong@redhat.com>