KVM: emulator: do not needlesly sync registers from emulator ctxt to vcpu

Currently we sync registers back and forth before/after exiting
to userspace for IO, but during IO device model shouldn't need to
read/write the registers, so we can as well skip those sync points. The
only exaception is broken vmware backdor interface. The new code sync
registers content during IO only if registers are read from/written to
by userspace in the middle of the IO operation and this almost never
happens in practise.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -385,6 +385,8 @@ struct kvm_vcpu_arch {
 	/* emulate context */
 
 	struct x86_emulate_ctxt emulate_ctxt;
+	bool emulate_regs_need_sync_to_vcpu;
+	bool emulate_regs_need_sync_from_vcpu;
 
 	gpa_t time;
 	struct pvclock_vcpu_time_info hv_clock;