qemu.tests: Update hdparm to work with old guests

In the old guests, hdparm command line always exits with status 0.
So remove the status check in the code. And also add a count to
caculate the unsupport parameters number. If all parameters are
not supported, fail the case with TestNAError.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #43 from ypu/hdparm

qemu.tests: Update hdparm to work with old guests