vnc: add error propagation to vnc_display_open

Before:

    $ qemu-system-x86_64 -vnc foo.bar:12345
    getaddrinfo(foo.bar,18245): Name or service not known
    Failed to start VNC server on `foo.bar:12345'

    $ qemu-system-x86_64 -vnc localhost:12345,reverse=on
    inet_connect_opts: connect(ipv4,yakj.usersys.redhat.com,127.0.0.1,12345): Connection refused
    Failed to start VNC server on `localhost:12345,reverse=on'

After:

    $ x86_64-softmmu/qemu-system-x86_64 -vnc foo.bar:12345
    Failed to start VNC server on `foo.bar:12345': address resolution failed for foo.bar:18245: Name or service not known

    $ x86_64-softmmu/qemu-system-x86_64 -vnc localhost:12345,reverse=on
    Failed to start VNC server on `localhost:12345,reverse=on': Failed to connect to socket: Connection refused

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