xen_xs: Guard against set but empty kernel argument

xen_xs: Guard against set but empty kernel argument

On xen 4.1 I observed configurations that look like:

(image
    (hvm
        (kernel '')
        (loader '/foo/bar')
))

The kernel element is there but unset. This leads to an empty <kernel/>
element in the XML and even worse makes us skip the boot order parsing
and therefore not emit a <boot device='$dev>'/> element which breaks CD
booting.