Ignore backing file errors in FS storage pool

Ignore backing file errors in FS storage pool

Currently a single storage volume with a broken backing file will disable the
whole storage pool. This can happen when the backing file is on some
unavailable network storage or if the backing volume is deleted, while the
storage volumes using it remain.
Since the storage pool can not be re-activated, re-creating the missing
or deleting the now useless volumes using libvirt only is not possible.

Fixing this is a little bit tricky:
1. virStorageBackendProbeTarget() only detects the missing backing file,
   if the backing file format is not explicitly specified. If the
   backing file is created using
	   kvm-img create -f qcow2 -o backing_fmt=qcow2,backing_file=... ...
   no error is detected at this stage.
   The new return code -3 signals that the backing file could not be
   opened.
2. The backingStore.format must be >= 0, since values < 0 would break
   virStorageVolTargetDefFormat() when dumping the XML data such as
       <format type='...'/>
   Because of this the format is faked as VIR_STORAGE_FILE_RAW.
3. virStorageBackendUpdateVolTargetInfo() always opens the backing file
   and thus always detects a missing backing file.
   Since it "only" updates the capacity, allocation, owner, group, mode
   and SELinux label, just ignore errors at this stage, print an error
   message and continue.
4. Using vol-dump on a broken volume still doesn't work, but at least
   vol-destroy and pool-refresh do work now.

To reproduce:
  dir=$(mktemp -d)
  virsh pool-create-as tmp dir '' '' '' '' "$dir"
  virsh vol-create-as --format qcow2 tmp back 1G
  virsh vol-create-as --format qcow2 --backing-vol-format qcow2 --backing-vol back tmp cow 1G
  virsh vol-delete --pool tmp back
  virsh pool-refresh tmp
After the last step, the pool will be gone (because it was not persistent). As
long as the now broken image stays in the directory, you will not be able to
re-create or re-start the pool.

Signed-off-by: Philipp Hahn <hahn@univention.de>