xen: Simplify halting of first CPU

Use the global first_cpu variable to halt the CPU rather than using a
local first_cpu initialized from qemu_get_cpu(0).

This will allow to change qemu_get_cpu() return type to CPUState
despite use of the CPU_COMMON halted field in the reset handler.

Signed-off-by: Andreas Färber <afaerber@suse.de>

xen-all.c

@@ -585,9 +585,7 @@ static void xen_reset_vcpu(void *opaque)
 
 void xen_vcpu_init(void)
 {
-    CPUArchState *first_cpu;
-
-    if ((first_cpu = qemu_get_cpu(0))) {
+    if (first_cpu != NULL) {
         qemu_register_reset(xen_reset_vcpu, first_cpu);
         xen_reset_vcpu(first_cpu);
     }