vmm vmx: Add a global bool to indicate if the host has the TSC_AUX MSR.

vmm vmx: Add a global bool to indicate if the host has the TSC_AUX MSR.

A future commit will remove direct access to vCPU structures from
struct vmx, so add a dedicated boolean for this rather than checking
the capabilities for vCPU 0.

Reviewed by:	corvink, markj
Differential Revision:	https://reviews.freebsd.org/D37269