When using a Secondary Bus Reset, all devices on the bus are reset.

Extend the pciResetDevice() API so that a 'check' callback can be
supplied which will verify that it is safe to reset the other devices
on the bus.

The virDomainObjPtr parameter is needed so that when the check function
iterates over the domain list, it can avoid double locking.

* src/pci.[ch]: add a 'check' callback to pciResetDevice(), re-work
  pciIterDevices() to pass the check function to the iter functions,
  use the check function in the bus iterator, return the first unsafe
  device from pciBusCheckOtherDevices() and include its details in
  the bus reset error message.

* src/qemu_driver.c, src/xen_uninified.c: just pass NULL as the
  check function for now