bhyve: Sleep briefly in the VMEXIT_DEBUG handler

As of commit 0bda8d3e9f ("vmm: permit some IPIs to be handled by
userspace") and commit 9cc9abf409 ("bhyve: create all vcpus on
startup"), we have a misbehaviour where AP vCPU threads spin until they
receive a SIPI.  In particular, since they are "suspended", they simply
call the VMEXIT_DEBUG handler in a loop, but the handler is a no-op by
default.

This is tricky to fix since the gdb stub isn't aware of whether a given
vCPU is supposed to be running.  For 13.2's sake, introduce a simple
workaround wherein the VMEXIT_DEBUG handler sleeps for a short period.
This ensures that host CPU usage remains sane when VMs are starting
without penalizing users of VMEXIT_DEBUG too much.

Reviewed by:	corvink, jhb
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D39174

usr.sbin/bhyve/bhyverun.c

@@ -911,6 +911,11 @@ vmexit_debug(struct vmctx *ctx __unused, struct vm_exit *vme __unused,
 #ifdef BHYVE_SNAPSHOT
 	checkpoint_cpu_resume(*pvcpu);
 #endif
+	/*
+	 * 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);
 }