KVM: PPC: Book3S HV: Fix incorrect userspace exit on ioeventfd write

When the guest does an MMIO write which is handled successfully by an
ioeventfd, ioeventfd_write() returns 0 (success) and
kvmppc_handle_store() returns EMULATE_DONE.  Then
kvmppc_emulate_mmio() converts EMULATE_DONE to RESUME_GUEST_NV and
this causes an exit from the loop in kvmppc_vcpu_run_hv(), causing an
exit back to userspace with a bogus exit reason code, typically
causing userspace (e.g. qemu) to crash with a message about an unknown
exit code.

This adds handling of RESUME_GUEST_NV in kvmppc_vcpu_run_hv() in order
to fix that.  For generality, we define a helper to check for either
of the return-to-guest codes we use, RESUME_GUEST and RESUME_GUEST_NV,
to make it easy to check for either and provide one place to update if
any other return-to-guest code gets defined in future.

Since it only affects Book3S HV for now, the helper is added to
the kvm_book3s.h header file.

We use the helper in two places in kvmppc_run_core() as well for
future-proofing, though we don't see RESUME_GUEST_NV in either place
at present.

[paulus@samba.org - combined 4 patches into one, rewrote description]

Suggested-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>