Implement AMD SEV support in vmd(8).

Implement AMD SEV support in vmd(8).

To launch a guest with AMD SEV enabled, vmd needs to do a few things:
- retrieve ASID used by guest on VM creation
- provide ASID to psp(4)
- let psp(4) encrypt memory used intially by guest
- run guest
- release resources held by psp(4) on guest shutdown
To enable SEV for a guest use the parameter "sev" in the guest's vm
section in vm.conf.

from hshoexer@; OK mlarkin@