xen-pcifront: Sanity check the MSI/MSI-X values

Check the returned vector values for any values that are
odd or plain incorrect (say vector value zero), and if so
print a warning. Also fixup the return values.

This patch was precipiated by the Xen PCIBack returning the
incorrect values due to how it was retrieving PIRQ values.
This has been fixed in the xen-pciback by
"xen/pciback: Utilize 'xen_pirq_from_irq' to get PIRQ value"
patch.

Reviewed-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>