vmm vmx: Allocate vpids on demand as each vCPU is initialized.

Compared to the previous version this does mean that if the system as
a whole runs out of dedicated vPIDs you might end up with some vCPUs
within a single VM using dedicated vPIDs and others using shared
vPIDs, but this should not break anything.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37169

(cherry picked from commit 58eefc67a1cf16623c23354efd089f65401c0455)