KVM: VMX: raise internal error for exception during invalid protected mode state

Exit to userspace with KVM_INTERNAL_ERROR_EMULATION if we encounter
an exception in Protected Mode while emulating guest due to invalid
guest state.  Unlike Big RM, KVM doesn't support emulating exceptions
in PM, i.e. PM exceptions are always injected via the VMCS.  Because
we will never do VMRESUME due to emulation_required, the exception is
never realized and we'll keep emulating the faulting instruction over
and over until we receive a signal.

Exit to userspace iff there is a pending exception, i.e. don't exit
simply on a requested event. The purpose of this check and exit is to
aid in debugging a guest that is in all likelihood already doomed.
Invalid guest state in PM is extremely limited in normal operation,
e.g. it generally only occurs for a few instructions early in BIOS,
and any exception at this time is all but guaranteed to be fatal.
Non-vectored interrupts, e.g. INIT, SIPI and SMI, can be cleanly
handled/emulated, while checking for vectored interrupts, e.g. INTR
and NMI, without hitting false positives would add a fair amount of
complexity for almost no benefit (getting hit by lightning seems
more likely than encountering this specific scenario).

Add a WARN_ON_ONCE to vmx_queue_exception() if we try to inject an
exception via the VMCS and emulation_required is true.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>