vmm: Don't lock a vCPU for VM_PPTDEV_MSI[X].

These are manipulating state in a ppt(4) device none of which is
vCPU-specific. Mark the vcpu fields in the relevant ioctl structures
as unused, but don't remove them for now.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37639