Do a proper emulation of guest writes to MSR_EFER.

• Must-Be-Zero bits cannot be set.
• EFER_LME and EFER_LMA should respect the long mode consistency checks.
• EFER_NXE, EFER_FFXSR, EFER_TCE can be set if allowed by CPUID capabilities.
• Flag an error if guest tries to set EFER_LMSLE since bhyve doesn't enforce segment limits in 64-bit mode.

MFC after: 2 weeks