netinfo:nicSpeed(): fix nicSpeed condition

In `if s not in (2 ** 16 - 1, 2 ** 32 - 1) or s > 0` first part doesn't
make sense with OR. Changed to AND.

I changed conditions to make the function more readable and created unit
test, to be sure that function returns correct values.

Change-Id: I34cfca16909f9695441e26d3ddd508e7a4210c12
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1157224
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34694
Reviewed-by: Dan Kenigsberg <danken@redhat.com>