Add support for the IA32_FEATURE_CONTROL MSR in bhyve
Certain guests (so far it's been observed with recent releases of Windows 10) attempt to read the
IA32_FEATURE_CONTROL msr which is involved with the Intel SGX support and related features (per the Intel SDM vol4). We currently do not support this MSR, requiring the
-w flag when running bhyve with such a guest.
Since we don't currently support SGX features in the guest, we can return
1 which indicates this MSR is locked and no further writes should be attempted (and with all features disabled).