libxl: fix vfb related assertion problem when starting pv-domU

libxl: fix vfb related assertion problem when starting pv-domU

When a config-file for a pv-domU has a vfb section with a vnc=0
option then a "xl create" for that domU crashes with the message

xl: libxl.c:252: libxl_defbool_val: Assertion `!libxl_defbool_is_default(db)'
failed.

and the domU hangs early in the start.

This patch fixes the cause of the assertion and let the domU start
normally.

This problem exists since xen 4.2.

Signed-off-by: Ronny Hegewald <Ronny.Hegewald@online.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>