pci: make virPCIDeviceReset more autonomous

pci: make virPCIDeviceReset more autonomous

I recently patches the callers to virPCIDeviceReset() to not call it
if the current driver for a device was vfio-pci (since that driver
will always reset the device itself when appropriate. At the time, Dan
Berrange suggested that I could instead modify virPCIDeviceReset
to check the currently bound driver for the device, and decide
for itself whether or not to go ahead with the reset.

This patch removes the previously added checks, and replaces them with
a check down in virPCIDeviceReset(), as suggested.

The functional difference here is that previously we were deciding
based on either the hostdev configuration or the value of
stubDriverName in the virPCIDevice object, but now we are actually
comparing to the "driver" link in the device's sysfs entry
directly. In practice, both should be the same.