libvirt.tests: Adjust testcases for timer test module.

1.Block no_timer should not work under banned_timer because
  no timer is banned for test.
2.In the four timers, only kvm-clock works.Other allowed timers
  are: pit, rtc and hypervclock
  (Leave it FAIL because RHEL6 manual does not point it out)

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>