xen: events: refactor xen_create_msi_irq slightly

Calling PHYSDEVOP_map_pirq earlier simplifies error handling and
starts to make the tail end of this function look like
xen_bind_pirq_msi_to_irq.

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