Introduce qemuMonitorTransactionBitmapMergeSourceAddBitmap which adds
the appropriate entry into a virJSONValue array to be used with
qemuMonitorTransactionBitmapMerge. Bitmap merging supports two possible
formats and this new helper implements the more universal one specifying
also the source node name.

In addition use the new helper in the testQemuMonitorJSONTransaction
test.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>