KVM: nVMX: Fail on MMIO completion for nested posted interrupts

When the kernel has no mapping for the vmcs02 virtual APIC page,
userspace MMIO completion is necessary to process nested posted
interrupts. This is not a configuration that KVM supports. Rather than
silently ignoring the problem, try to exit to userspace with
KVM_INTERNAL_ERROR.

Note that the event that triggers this error is consumed as a
side-effect of a call to kvm_check_nested_events. On some paths
(notably through kvm_vcpu_check_block), the error is dropped. In any
case, this is an incremental improvement over always ignoring the
error.

Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-7-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>