pci: Do not check if a bus exist in pci_parse_devaddr.

Actually, pci_parse_devaddr checks if the dom/bus of the PCI address exist. But
this should be the jobs of a caller. In fact, the two callers of this function
will try to retrieve the PCIBus related to the devaddr and return an error if
they cannot.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
