When doing block commit we need to allow write for members of the
backing chain so that we can commit the data into them.

qemuDomainDiskChainElementPrepare was used for this which since commit
786d8d91 calls qemuDomainNamespaceSetupDisk which has very adverse
side-effects, namely it relabels the nodes to the same label it has in
the main namespace. This was messing up permissions for the commit
operation since its touching various parts of a single backing chain.

Since we are are actually not introducing new images at that point add a
flag for qemuDomainDiskChainElementPrepare which will refrain from
calling to the namespace setup function.

Calls from qemuDomainSnapshotCreateSingleDiskActive and
qemuDomainBlockCopyCommon do introduce new members all calls from
qemuDomainBlockCommit do not, so the calls are anotated accordingly.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1506072