xen/pci: Used cached MSI-X capability offset

We now cache the MSI-X capability offset in the struct pci_dev, so no
need to find the capability again.

Acked-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>