KVM: x86: Check for injected exceptions before queuing a debug exception

KVM: x86: Check for injected exceptions before queuing a debug exception

On KVM_SET_GUEST_DEBUG, if a #DB or #BP is injected with
KVM_GUESTDBG_INJECT_DB or KVM_GUESTDBG_INJECT_BP, KVM fails with -EBUSY
if there is an existing pending exception. This was introduced in
commit 4f926bf29186 ("KVM: x86: Polish exception injection via
KVM_SET_GUEST_DEBUG") to avoid a warning in kvm_queue_exception(),
presumably to avoid overriding a pending exception.

This added another (arguably nice) property, if there's a pending
exception, KVM_SET_GUEST_DEBUG cannot cause a #DF or triple fault.
However, if an exception is injected, KVM_SET_GUEST_DEBUG will cause
a #DF or triple fault in the guest, as kvm_multiple_exception() combines
them.

Check for both pending and injected exceptions for
KVM_GUESTDBG_INJECT_DB and KVM_GUESTDBG_INJECT_BP, to avoid accidentally
injecting a #DB or triple fault.

Signed-off-by: Yosry Ahmed <yosry@kernel.org>
base-commit: a68a4bbc5b9ce5b722473399f05cb05217abaee8
Signed-off-by: Sean Christopherson <seanjc@google.com>