QA, burnin: allow selection of reboot types

After some more investigation, only the soft reboot type fails for Xen
3.4 (due to the reboot/uptime time counter). As such, it's better to
allow selective testing, since we do want to test in general these
opcodes/the command line script.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Luca Bigliardi <shammash@google.com>