Allow creation of user creatable object types with qemu-img
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-img info --object secret,id=sec0,file=mypasswd.txt \
      ...other info args...
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>