virttest.utils_libguestfs: Add sets of commands for is-*
virttest.utils_libguestfs: Add cmd_result() in GuestfishRemote

1. Want keep consistent with the original code.
add method cmd_result() in class GuestfishRemote(), cmd_result is
called by inner_cmd()

2. add a func blockdev_rereadpt()
virttest.utils_libguestfs: Add two API interface

1. max-disks
2. nr-devices
Merge pull request #1617 from vanbogithub/bfan-next

virttest.utils_libguestfs: Add sets of commands for is-*