qemu: Introduce new set of helpers for attaching and detaching storage chains

qemu: Introduce new set of helpers for attaching and detaching storage chains

These are meant to replace the ad-hoc helpers qemuHotplugDiskSourceAttach...
and the open-coded version in qemu_command.c for use in command line
generation.

The functions for preparing for attach of chains unfortunately need to
be in qemu_command.c as they use function defined by that file and
inclusion hierarchy.

In this patch new functions are introduced and subsequent patches then
refactor individual parts to use them.

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