tests: kvm: Check for a kernel warning

When running with /sys/module/kvm_intel/parameters/unrestricted_guest=N,
test that a kernel warning does not occur informing us that
vcpu->mmio_needed=1.  This can happen when KVM_RUN is called after a
triple fault.
This test was made to detect a bug that was reported by Syzkaller
(https://groups.google.com/forum/#!topic/syzkaller/lHfau8E3SOE) and
fixed with commit bbeac283

 ("KVM: X86: Fix residual mmio emulation
request to userspace").

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>