vmm: Lookup vcpu pointers in vmmdev_ioctl.

Centralize mapping vCPU IDs to struct vcpu objects in vmmdev_ioctl and
pass vcpu pointers to the routines in vmm.c. For operations that want
to perform an action on all vCPUs or on a single vCPU, pass pointers
to both the VM and the vCPU using a NULL vCPU pointer to request
global actions.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37168