guestfish_block_dev: Add two cases for test API

1. blockdev-rereadpt
2. canonical-device-name
guestfish_block_dev: Add three cases for test API

1. device-index
2. list-devices
3. disk-format
guestfish_block_dev: Add three cases to test API

1. max-disks
2. nr-devices
3. list-partitions
guestfish_block_dev: Add two cases to test API

1. disk-has-backing-file
2. disk-virtual-size