Fix get_disk_size to handle partitions

Rename to get_block_size, as well.

Resolves 27.

Signed-off-by: Andy Grover <agrover@redhat.com>
Rename get_block_* to get_blockdev_*

This makes it clear we're getting the size of the block device, and not how
big the block size is.

Inline get_block_numbers into get_blockdev_type.

Alias new names to old names for b/w compat with targetcli.

Signed-off-by: Andy Grover <agrover@redhat.com>