vditest: fix vdisize handling

This fixes a bug introduced by commit 0b37b3.  'qemu-io -c length'
returns size with "iB" (e.g. KiB, MiB, GiB).  This handles it
correctly.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Makefile: add target to check coding style

This enables us to run checkpatch.pl against all the source files with
'make check-style'.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>