xen/pci: Bubble up error and fix description.

The function is never called under PV guests, and only shows up
when MSI (or MSI-X) cannot be allocated. Convert the message
to include the error value.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>