qemu-nbd: add support for --object command line arg

qemu-nbd: add support for --object command line arg

Allow creation of user creatable object types with qemu-nbd
via a new --object command line arg. This will be used to supply
passwords and/or encryption keys to the various block driver
backends via the recently added 'secret' object type.

 # printf letmein > mypasswd.txt
 # qemu-nbd --object secret,id=sec0,file=mypasswd.txt \
      ...other nbd args...

Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Daniel P. Berrange <[email protected]>
Message-Id: <1455129674[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>