vmm: enable software breakpoints for AMD CPUs

[bnovkov]

Summary

This patch adds support for software breakpoint vmexits on AMD SVM.
It implements the VM_CAP_BPT_EXIT used to enable software breakpoints. When enabled, breakpoint vmexits are passed to userspace where they are handled by the GDB stub.
It is the first of five patches that enable single-stepping virtual machines on AMD hosts.

This work was sponsored by the GSoC '22 program.

[corvink]

Maybe add some more information to the commit message. Something like "This commit adds a new capability to enable software breakpoints. When enabled, vmexits are passed to userspace where they should be handled. ..."

sys/amd64/vmm/amd/svm.c

1453

Ideally should be an additional patch. See https://reviews.freebsd.org/D42298#inline-257920 for some reasons.

[bnovkov]

Address @corvink's comments, unrelated exception reflection change was split into its own patch (D42405), updated commit message to be more clear.

[jhb]

sys/amd64/vmm/amd/svm.c

1429

Dropping this (merging the reflect fix first so then this isn't needed)