vmm: refactor event reflection in AMD SVM

This patch refactors AMD SVM event reflection to allow events to be
propagated to userland, rather than always reflected into the guest.

This is necessary to implement some capabilities that request VMEXITs
when a specific exception occurs (e.g. VM_CAP_BPT_EXIT).

Reviewed by: jhb
Sponsored by: Google, Inc. (GSoC 2022)
Differential Revision: https://reviews.freebsd.org/D42405