Expose max_vcpus in CPUID for HVM guests

Gerd Hoffman and Anthony Perard mentioned in the edk2-devel mailing list that it would be helpful for Xen to provide access to the maximum number of vCPUs so OVMF gets a chance to set itself up appropriately at runtime rather than through a compile time constant. See the thread here:

https://edk2.groups.io/g/devel/topic/105721898#msg118247

Seems plausible to expose max_vcpus in leaf 0x40000005:edx (0x40000105:edx under viridian) and allow more vCPUs than the current hard coded limit of 64. And future-proof it by allowing OVMF to self-configure.