virhostdevtest: Decrease possibility of uninitialized @subsys

virhostdevtest: Decrease possibility of uninitialized @subsys

With the current way the myInit() is written, it's fairly easy to
miss initialization of @subsys variable as the variable is
allocated firstly on the stack and then it's assigned to
hostdev[i] which was allocated using g_new0() (this it is
containing nothing but all zeroes).

Make the subsys point to the corresponding member in hostdev[i]
from the start. This way only the important bits are overwritten
and the rest stays initialized to zero.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>