xen/pcifront: avoid pci_frontend_enable_msix() falsely returning success

The original XenoLinux code has always had things this way, and for
compatibility reasons (in particular with a subsequent pciback
adjustment) upstream Linux should behave the same way (allowing for two
distinct error indications to be returned by the backend).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>