libxl_conf: Add check for unsupported graphics type

libxl_conf: Add check for unsupported graphics type

libxlMakeVfb always succeeds regardless of if the graphics type is
actually supported or not.

libxl_defbool_val is called in libxlMakeBuildInfoVfb which besides returning
the boolean value of the defbool also has an assertion that the defbool value
is not set to default. It is possible to fail this assertion if an
unsupported graphics type is used. In libxlMakeVfb, the VNC and SDL enable
defbools are still left in their default state if the graphics type falls
outside the two, which leads to this issue.

This patch adds a check to reject graphics types outside of SDL, VNC, and SPICE
very early on in libxlMakeVfb. As a safeguard, we also initialize both vnc
enable and sdl enable defbools as false early.

Signed-off-by: Rayhan Faizel <rayhan.faizel@gmail.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>