xen/arm: Fix unreachable panic for vpl011 vIRQ

When creating direct mapped domU, the vIRQ for vpl011 is taken from
the SERHND_DTUART serial port using serial_irq. This function can return
-1 (i.e. no interrupt found) in which case we should call a panic.
However, vpl011_virq is defined as unsigned int which causes the panic
to be unreachable, because less than zero comparison of an unsigned value
can never be true. Fix it by defining vpl011_virq as int.

Fixes: 3580c8b2df ("xen/arm: if direct-map domain use native UART address and IRQ number for vPL011")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/domain_build.c

@@ -3897,7 +3897,7 @@ void __init create_domUs(void)
 
         if ( !dt_property_read_u32(node, "nr_spis", &d_cfg.arch.nr_spis) )
         {
-            unsigned int vpl011_virq = GUEST_VPL011_SPI;
+            int vpl011_virq = GUEST_VPL011_SPI;
 
             d_cfg.arch.nr_spis = gic_number_lines() - 32;