Make get_blockdev_size work better with md

Not all block device names that end with a number are partitions. Rewrite
the function so we only look for partitions (by looking for ending number)
if just reading /sys/block/foo/size fails.

Reported-By: Craig Watson <craig.watson@vanguard-rugged.com>
Signed-off-by: Andy Grover <agrover@redhat.com>