KVM: x86 emulator: Fix popf emulation

POPF behaves differently depending on current CPU mode. Emulate correct
logic to prevent guest from changing flags that it can't change otherwise.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Avi Kivity <avi@redhat.com>