ui: fix regression in x509verify parameter for VNC server

ui: fix regression in x509verify parameter for VNC server

The 'x509verify' parameter is documented as taking a path to the
x509 certificates, ie the same syntax as the 'x509' parameter.

  commit 4db14629c38611061fc19ec6927405923de84f08
  Author: Gerd Hoffmann <[email protected]>
  Date:   Tue Sep 16 12:33:03 2014 +0200

    vnc: switch to QemuOpts, allow multiple servers

caused a regression by turning 'x509verify' into a boolean
parameter instead. This breaks setup from libvirt and is not
consistent with the docs.

Signed-off-by: Daniel P. Berrange <[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>