bhyve: Sleep for a short period after VM_EXITCODE_DEBUG exits

Apply 2e2b8afa52 for RISC-V:

As on amd64 and aarch64 APs will repeatedly exit until they are
brought online, so this hack helps avoid burning CPU time during
guest bootup.

Reviewed by:	corvink, markj
Differential Revision:	https://reviews.freebsd.org/D48811

usr.sbin/bhyve/riscv/vmexit.c

@@ -47,6 +47,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include <vmmapi.h>
 
@@ -135,6 +136,12 @@ vmexit_debug(struct vmctx *ctx __unused, struct vcpu *vcpu __unused,
     struct vm_run *vmrun __unused)
 {
 
+	/*
+	 * XXX-MJ sleep for a short period to avoid chewing up the CPU in the
+	 * window between activation of the vCPU thread and the
+	 * SBI_HSM_HART_START request.
+	 */
+	usleep(1000);
 	return (VMEXIT_CONTINUE);
 }