Enable AMD SEV support in vmm(4).

Enable AMD SEV support in vmm(4).

Bring the pieces for vmm(4) to support guests with SEV memory
encryption on AMD CPUs.  The corresponding vmd(8) changes will
follow.
Emulate cpuid 0x8000001f so the guest can discover SEV features.
Allow vmd(8) to enable SEV on VM creation.  Inform vmd(8) about the
c-bit position and ASID assigned to each VCPU.
Note that vmd(8) has to be rebuilt with the new header files.

from hshoexer@; input dv@; OK mlarkin@