i386/kvm: use a switch statement for MSR detection

Switch is easier on the eye and might lead to better codegen.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170807085703.32267-2-lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>