amd64: Fix sys/pcpu.h usage in vmm_host.h and md_var.h

Include sys/pcpu in vmm_host.h as its structs and functions are used
there, and add a forward declaration of struct pcpu to md_var.h as it
is used in some function prototypes.

Reviewed by: corvink, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51550