bhyve: implement rdmsr for MSR_IA32_FEATURE_CONTROL

Without the -w option, Windows guests crash on boot. This is caused by a rdmsr
of MSR_IA32_FEATURE_CONTROL. Windows checks this MSR to determine enabled VMX
features. This MSR isn't emulated in bhyve, so a #GP exception is injected
which causes Windows to crash.

Fix by returning a rdmsr of MSR_IA32_FEATURE_CONTROL with Lock Bit set and
VMX disabled to informWindows that VMX isn't available.

Reviewed by: jhb, grehan (bhyve)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29665