qemu: Add test kernbench2

This case is used for performance testing.

Changes for V2
1. Only update gcc when testing on host.
2. Add record_result() function to record the result value to a file.
Changes for V3:
Clean the folder used to make kernel

Signed-off-by: Feng Yang <fyang@redhat.com>
Acked-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Amos Kong <akong@redhat.com>
qemu.tests.kexec: Fix a couple of problems found by pylint

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