qemuMigrationSrcNBDCopyCancel: Use qemuBlockStorageSourceAttachRollback to detach...

qemuMigrationSrcNBDCopyCancel: Use qemuBlockStorageSourceAttachRollback to detach migration NBD blockdevs

Rewrite the code to use the common tooling for removing blockdevs
instead of the ad-hoc qemuBlockStorageSourceDetachOneBlockdev helper.

Use of the common infrastructure will properly handle cases when the raw
driver is ommited from the block graph.

Since the TLS data object is shared for all migration QMP commands and
objects we need to strip its alias from the definition of the storage
source before attempting to detach it.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>