KVM: x86: let reexecute_instruction work for tdp

KVM: x86: let reexecute_instruction work for tdp

Currently, reexecute_instruction refused to retry all instructions if
tdp is enabled. If nested npt is used, the emulation may be caused by
shadow page, it can be fixed by dropping the shadow page. And the only
condition that tdp can not retry the instruction is the access fault
on error pfn

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>