The routine kvmppc_set_passthru_irq() calls kvmppc_xive_set_mapped()
and kvmppc_xive_clr_mapped() with an IRQ descriptor. Use directly the
host IRQ number to remove a useless conversion.

Add some debug.
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-15-clg@kaod.org