vhost-user: only initialize queue 0 CharBackend

All the queues share the same chardev. Initialize only the first queue
CharBackend, and pass it to other queues. This will allow to claim the
chardev only once in a later change.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-18-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
