Fix "Usage" to work with debian system.

Failed to start OSv based on README.md with debian/jessie as follows.

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sat@glenda:~/src/capstan$ ~/bin/capstan run -n bridge -b virbr0 cloudius/osv
Created instance: cloudius-osv
failed to launch bridge helper
qemu-system-x86_64: -netdev bridge,id=hn0,br=virbr0,helper=/usr/libexec/qemu-bridge-helper: Device 'bridge' could not be initialized

exit status 1
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It's because debian doesn't have qemu-bridge-helper script. We should run
it as follows instead.

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sat@glenda:~$ bin/capstan run -b virbr0 cloudius/osv
Created instance: cloudius-osv
OSv v0.09
eth0: 192.168.122.15

[/]%
===============================================================================

We should manually create bridge (in this case, virbr0) beforehand.

Reviewed-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>