conf: introduce 'msrs' feature

conf: introduce 'msrs' feature

Introduce the 'msrs' feature element that controls Model Specific
Registers related behaviour. At this moment it allows only
single tunable attribute "unknown":

 <msrs unknown='ignore|fault'/>

Which tells hypervisor to ignore accesses to unimplemented
Model Specific Registers. The only user of that for now is going
to be the bhyve driver.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>