muen-sinfo: Provide muen_get_tsc_khz getter function
muen-sinfo: Make check_magic an API function

The muen_check_magic function allows external code to check if the Muen
sinfo page is present.
Add Muen hypervisor detection code

The detection mechanism checks if the sinfo page magic is present.

If the Muen hypervisor is detected, override the native calibrate_tsc
function of the x86 platform with a paravirtualized version which reads
the TSC tick rate in kHz from the sinfo page.
Add MUEN_GUEST config option to arch/x86/Kconfig

Select this option to enable Muen hypervisor detection.
Build Muen detection code if MUEN_GUEST is enabled
Add x86_hyper_muen struct to known hypervisors

Only try to detect the Muen hypervisor if MUEN_GUEST is enabled.