i386: hvf: Make long mode enter and exit clearer

i386: hvf: Make long mode enter and exit clearer

Intel SDM "9.8.5 Initializing IA-32e Mode" and "9.8.5.4 Switching Out of
IA-32e Mode Operation" define activation and deactivation of long mode
only upon a change of CR0.PG but current code invokes exit_long_mode()
unconditionally until LME is cleared.

Signed-off-by: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200630102824.77604-6-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>