arm64/vmm: Update exception vectors around a guest

When entering into a guest with VHE we need to switch from the kernel
exception vectors to the vmm exception vectors. The latter understands
an exception will be from a guest and can switch back to a kernel
context.

Rather than encoding the location of the kernel vectors we can just
read the value from vbar_el2 and restore it later.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46081