qemu-storage-daemon: Add main loop

qemu-storage-daemon: Add main loop

Instead of exiting after processing all command line options, start a
main loop and keep processing events until exit is requested with a
signal (e.g. SIGINT).

Now qemu-storage-daemon can be used as an alternative for qemu-nbd that
provides a few features that were previously only available from QMP,
such as access to options only available with -blockdev and the socket
types 'vsock' and 'fd'.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-13-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>