virttest.virsh: add blkiotune command wrapper

Signed-off-by: Alex Jia <ajia@redhat.com>
virttest.utils_misc: add functions for getting major and minor numbers of the device

Signed-off-by: Alex Jia <ajia@redhat.com>
virttest.libvirt_xml.vm_xml: parse block I/O elements from XML

Signed-off-by: Alex Jia <ajia@redhat.com>
Merge pull request #1678 from chuanchang/blkiotune

Add helper functions for testing virsh blkiotune command