xen: fix error handling path if xen_allocate_irq_dynamic fails

It is possible that the call to xen_allocate_irq_dynamic() returns negative
number other than -1.

Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>