powerpc/kvm: Fix guest boot failure on Power9 since DAWR changes

SLOF checks for 'sc 1' (hypercall) support by issuing a hcall with
H_SET_DABR. Since the recent commit e8ebedbf ("KVM: PPC: Book3S
HV: Return error from h_set_dabr() on POWER9") changed H_SET_DABR to
return H_UNSUPPORTED on Power9, we see guest boot failures, the
symptom is the boot seems to just stop in SLOF, eg:

  SLOF ***************************************************************
  QEMU Starting
   Build Date = Sep 24 2017 12:23:07
   FW Version = buildd@ release 20170724
  <no further output>

SLOF can cope if H_SET_DABR returns H_HARDWARE. So wwitch the return
value to H_HARDWARE instead of H_UNSUPPORTED so that we don't break
the guest boot.

That does mean we return a different error to PowerVM in this case,
but that's probably not a big concern.

Fixes: e8ebedbf

 ("KVM: PPC: Book3S HV: Return error from h_set_dabr() on POWER9")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>