KVM: x86: Assert that the emulator doesn't load CS with garbage in !RM

KVM: x86: Assert that the emulator doesn't load CS with garbage in !RM

Yell loudly if KVM attempts to load CS outside of Real Mode without an
accompanying control transfer type, i.e. on X86_TRANSFER_NONE.  KVM uses
X86_TRANSFER_NONE when emulating IRET and exceptions/interrupts for Real
Mode, but IRET emulation for Protected Mode is non-existent.  WARN instead
of trying to pass in a less-wrong type, e.g. X86_TRANSFER_RET, as
emulating IRET goes even beyond emulating FAR RET (which KVM also doesn't
fully support).

Reported-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Link: https://lore.kernel.org/r/20230216202254.671772-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>