qemu.tests: small bugs fix in multicase_iperf

Bugs fix include:
  - lines more than 80 letters
  - move clean_cmd to the beginning of the script as it is called
    in the finally
  - set ignore_status to True when try to get if host installed
    iperf

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #69 from ypu/small_fix

qemu.tests: small bugs fix in multicase_iperf