bhyve: Sleep briefly in the VMEXIT_DEBUG handler

As of commit 0bda8d3e9f7a ("vmm: permit some IPIs to be handled by
userspace") and commit 9cc9abf409cc ("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

(cherry picked from commit ef0ac973dbc8c54214ac8c61891abd832cd5b700)