xen: convert "-machine igd-passthru" to an accelerator property

xen: convert "-machine igd-passthru" to an accelerator property

The first machine property to fall is Xen's Intel integrated graphics
passthrough.  The "-machine igd-passthru" option does not set anymore
a property on the machine object, but desugars to a GlobalProperty on
accelerator objects.

The setter is very simple, since the value ends up in a
global variable, so this patch also provides an example before the more
complicated cases that follow it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>