qemu: blockjob: Ensure that config disk source is identical when modifying it

qemu: blockjob: Ensure that config disk source is identical when modifying it

qemuBlockJobRewriteConfigDiskSource rewrites the disk source only
according to the 'target'. This means that if someone would change the
inactive config of the VM to refer to a different disk a block job would
rewrite it when finishing a job which modifies the disk source.

Make sure that this does not happen by verifying that the source of the
config disk is the same.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
ACKed-by: Eric Blake <eblake@redhat.com>