Since commit c5f61513 qemuDomainBlockInfo tries to update the
"physical" storage size for all network storage and not only block
devices.

Since the storage driver APIs to do this are not implemented for certain
storage types (RBD, iSCSI, ...) the code would fail to retrieve any data
since the failure of qemuDomainStorageUpdatePhysical is fatal.

Since it's desired to return data even if the total size can't be
updated we need to ignore errors from that function and return plausible
data.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1442344