guestfish_block_dev: Split image preparation from individual test case
guestfish_block_dev: Add function test_blockdev_set_get_ro_rw()

To cover guestfish command:
(1)blockdev-setro
(2)blockdev-getro
(3)blockdev-setrw
Merge pull request #90 from vanbogithub/bfan-dev

guestfish_block_dev: Split image preparation from individual test case