detect-virt: Fix Xen domU discovery

The conditional for detection xen virtualization contained a little mistake.
It is checking for i to be empty: 'if (!i)  {', but it must check for cap instead,
because: 'cap = strsep(&i, ",")' will set cap to the discovered value and i to
the next value after the separator.
Hence, i would be empty, if there is only control_d in domcap, leading to a wrong
domU detection.

https://bugs.freedesktop.org/show_bug.cgi?id=77271