KVM: iommu: fix releasing unmapped page

There are two bugs:
- the 'error page' is forgot to be released
  [ it is unneeded after commit a2766325, for backport, we
    still do kvm_release_pfn_clean for the error pfn ]

- guest pages are always released regardless of the unmapped page
  (e,g, caused by hwpoison)

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>