bhyve: Sleep for a short period after VM_EXITCODE_DEBUG exits

As on amd64, APs will repeatedly exit until they are brought online, so this
hack helps avoid burning CPU time during guest bootup.

Reviewed by:	jhb
MFC after:	2 weeks
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D46195

usr.sbin/bhyve/aarch64/vmexit.c

@@ -43,6 +43,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include <vmmapi.h>
 
@@ -132,6 +133,11 @@ vmexit_debug(struct vmctx *ctx __unused, struct vcpu *vcpu,
     struct vm_run *vmrun __unused)
 {
 	gdb_cpu_suspend(vcpu);
+	/*
+	 * XXX-MJ sleep for a short period to avoid chewing up the CPU in the
+	 * window between activation of the vCPU thread and the STARTUP IPI.
+	 */
+	usleep(1000);
 	return (VMEXIT_CONTINUE);
 }