vmm svm: Refactor per-vCPU data.

• Allocate VMCBs separately to avoid excessive padding in struct svm_vcpu.

• Allocate APIC pages dynamically directly in struct vlapic.

• Move vm_mtrr into struct svm_vcpu rather than using a separate parallel array.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37148