Add binary option for command outputs collected by ovs-bugtool

Add binary option for command outputs collected by ovs-bugtool

Current ovs-bugtool collects command outputs as text strings.
Thus it reads the output by lines. For commands that generate
huge binary data, it becomes very inefficient to read the output.

The change here is to use a 1MB buffer to read binary data
instead of reading them by lines.

Signed-off-by: Shih-Hao Li <shihli@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>