Fix device assignment with root devices

Fix device assignment with root devices

The patches to add ACS checking to PCI device passthrough
introduced a bug.  With the current code, if you try to
passthrough a device on the root bus (i.e. bus 0), then
it denies the passthrough.  This is because the code in
pciDeviceIsBehindSwitchLackingACS() to check for a parent
device doesn't take into account the possibility of the
root bus.  If we are on the root bus, it means we
legitimately can't find a parent, and it also means that
we don't have to worry about whether ACS is enabled.
Therefore return 0 (indicating we don't lack ACS) from
pciDeviceIsBehindSwitchLackingACS().

Signed-off-by: Chris Lalancette <clalance@redhat.com>