bhyve: Sleep for a short period after VM_EXITCODE_DEBUG exits

Apply 2e2b8afa52da6 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