qemu: hotplug: Allow specifying explicit source for disk backend hotplug code
qemu: hotplug: Allow specifying explicit source for disk backend hotplug code

Since the code is also used when changing media we need to allow
specifying explicit source for which we are going to prepare. With this
change callers don't have to replace disk->src with the new source
definition for generating these.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

eskultety/libvirt,fabianfreyer/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,eskultety/libvirt,nertpinx/libvirt,jfehlig/libvirt,libvirt/libvirt,andreabolognani/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,fabianfreyer/libvirt,zippy2/libvirt,eskultety/libvirt,jardasgit/libvirt,crobinso/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,eskultety/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt,zippy2/libvirt,andreabolognani/libvirt,libvirt/libvirt,jardasgit/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,andreabolognani/libvirt,nertpinx/libvirt,eskultety/libvirt,zippy2/libvirt,jardasgit/libvirt,andreabolognani/libvirt