qemu-sockets: add error propagation to inet_dgram_opts

Before:

    $ qemu-system-x86_64 -monitor udp:localhost:631@localhost:631
    inet_dgram_opts: bind(ipv4,127.0.0.1,631): OK
    inet_dgram_opts failed
    chardev: opening backend "udp" failed

After:

    $ x86_64-softmmu/qemu-system-x86_64 -monitor udp:localhost:631@localhost:631
    qemu-system-x86_64: -monitor udp:localhost:631@localhost:631: Failed to bind socket: Address already in use
    chardev: opening backend "udp" failed

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>