target-i386: kvm: Simplify MSR setting functions

Simplify kvm_put_tscdeadline_msr() and
kvm_put_msr_feature_control() using kvm_msr_buf and the
kvm_msr_entry_add() helper.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>