Add test cases for blkiotune

This patches series are used for testing blkio parameters for a guest domain, it includes 68 test cases and covers positive & negative testing, please help review them.

v2~v1:
virttest/utils_misc.py: caught IOError instead of all of exception
virttest/libvirt_xml/vm_xml.py: changed 'virsh_instance=virsh' to last one in new_from_dumpxml()
                                            and updated get_blkio_params()
virsh_blkiotune.py: removed redundant codes and enhanced check on get_blkio_params_from_cgroup() 
                            and avoided naming confuse.
virsh_blkiotune.cfg: removed useless comment and vms line, and updated test cases 
                             based on previous comments

And also rebased codes based on latest libvirt test directory structure.