bhyve should expose lfence serialize in DE_CFG MSR
The DE_CFG MSR on modern AMD CPUs controls, among other things, whether the lfence instruction is serializing. While we cannot allow the guest to control the setting, we can at least expose whether it has been set (or not) in the host. Doing so will allow guests to safely use
lfence; rdtsc for performing a TSC reading when rdtscp is not available.