net: Only show vhost-user in the help text if CONFIG_POSIX is defined

According to net/Makefile.objs we only link in the vhost-user code
if CONFIG_POSIX has been set. So the help screen should also only
show this information if CONFIG_POSIX has been defined.
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>

qemu-options.hx

@@ -1998,8 +1998,10 @@ DEF("netdev", HAS_ARG, QEMU_OPTION_netdev,
     "                VALE port (created on the fly) called 'name' ('nmname' is name of the \n"
     "                netmap device, defaults to '/dev/netmap')\n"
 #endif
+#ifdef CONFIG_POSIX
     "-netdev vhost-user,id=str,chardev=dev[,vhostforce=on|off]\n"
     "                configure a vhost-user network, backed by a chardev 'dev'\n"
+#endif
     "-netdev hubport,id=str,hubid=n[,netdev=nd]\n"
     "                configure a hub port on QEMU VLAN 'n'\n", QEMU_ARCH_ALL)
 DEF("net", HAS_ARG, QEMU_OPTION_net,