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.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35623
Sponsored by: Beckhoff Automation GmbH & Co. KG