KVM: x86: clean up reexecute_instruction

Little cleanup for reexecute_instruction, also use gpa_to_gfn in
retry_instruction

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>