KVM: x86: work around leak of uninitialized stack contents

Emulation of VMPTRST can incorrectly inject a page fault
when passed an operand that points to an MMIO address.
The page fault will use uninitialized kernel stack memory
as the CR2 and error code.

The right behavior would be to abort the VM with a KVM_EXIT_INTERNAL_ERROR
exit to userspace; however, it is not an easy fix, so for now just ensure
that the error code and CR2 are zero.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Cc: stable@vger.kernel.org
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>