qemu: hotplug: Prepare disk source for media changing

qemu: hotplug: Prepare disk source for media changing

The disk storage source needs to be prepared if we want to use -blockdev
or secrets for the new media image. It does not hurt to do the same for
the legacy hotplug code as well.

Unfortunately helpers like qemuDomainPrepareDiskSource take
virDomainDiskDef as an argument and it would be hard to fix them to take
an explicit source, so the function also temporarily replaces disk->src
for the new source in this function.

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