xend: allow VM configuration with both sdl=1 and vnc=1

xend: allow VM configuration with both sdl=1 and vnc=1

In the stubdom case you can now specify sdl=1 and vnc=1 (again, both
can be selected at the same time) in the vfb configuration.
So instead of:

vfb = [ 'type=vnc' ]

now you have:

vfb = [ 'vnc=1,sdl=1' ]

the former configuration option is deprecated but still supported for
backward compatibility.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>