KVM: x86: emulator: Fix illegal LEA handling

The emulator mishandles LEA with register source operand. Even though such
LEA is illegal, it can be encoded and fed to CPU. In which case real
hardware throws #UD. The emulator, instead, returns address of
x86_emulate_ctxt._regs. This info leak hurts host's kASLR.

Tell the decoder that illegal LEA is not to be emulated.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Message-Id: <20220729134801.1120-1-mhal@rbox.co>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>