PCI: Add pci_probe_reset_slot() and pci_probe_reset_bus()

Users of pci_reset_bus() and pci_reset_slot() need a way to probe
whether the bus or slot supports reset.  Add trivial helper functions
and export them as vfio-pci will make use of these.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>