vmm: Add vm_gpa_hold_global wrapper function.

This handles the case that guest pages are being held not on behalf of
a virtual CPU but globally. Previously this was handled by passing a
vcpuid of -1 to vm_gpa_hold, but that will not work in the future when
vm_gpa_hold is changed to accept a struct vcpu pointer.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37160