qemu: block: Add helper to add common properties for -blockdev configuration

qemu: block: Add helper to add common properties for -blockdev configuration

The new helper replaces qemuBlockStorageSourceGetBlockdevFormatCommonProps
and the two inline instances generating the common properties for a
blockdev layer.

The new helper is to be used for both the format layer and the storage
backing layer, thus a new parameter 'effective' switches between the
modes.

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