Strip brackets from vnc host

Commit v2.2.0-1530-ge556032 vnc: switch to inet_listen_opts
bypassed the use of inet_parse in inet_listen, making literal
IPv6 addresses enclosed in brackets fail:

qemu-kvm: -vnc [::1]:0: Failed to start VNC server on `(null)': address
resolution failed for [::1]:5900: Name or service not known

Strip the brackets to make it work again.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>