vmm: permit some IPIs to be handled by userspace

Add VM_EXITCODE_IPI to permit returning unhandled IPIs to userland.
INIT and STARTUP IPIs are now returned to userland. Due to backward
compatibility reasons, a new capability is added for enabling
VM_EXITCODE_IPI.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D35623
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 0bda8d3e9f7a5c04881219723436616b23041e5f)