vmm vmx: Refactor per-vCPU data.

Add a struct vmx_vcpu to hold per-vCPU data specific to VT-x and
move parallel arrays out of struct vmx into a single array of
this structure.

While here, dynamically allocate the VMCS, APIC page and PIR
descriptors for each vCPU rather than embedding them.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37149

(cherry picked from commit 0f00260c679f8d192f9d673fe4fb94b47a2ac6c5)