virttest.utils_libguestfs.py: Add function do_mount()

Mount lvm or physical partation
virttest.utils_libguestfs: Add some blockdev commands of guestfish

(1)canonical_device_name
(2)device_index
(3)disk_format
(4)disk_has_backing_file
(5)disk_virtual_size
Merge pull request #1583 from vanbogithub/bfan-next

virttest.utils_libguestfs.py: Add function do_mount()