When initializing a libxl_domain_build_info struct with
libxl_domain_build_info_init(), VNC is enabled by default.  As a
result, VMs configured with no graphics still have VNC enabled.
This behavior is a regression wrt to the legacy Xen driver.
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>