For both AttachDevice and UpdateDevice APIs, if the disk device
is 'cdrom' or 'floppy', the operations could be ejecting, updating,
and inserting. For either ejecting or updating, the shared disk
entry of the original disk src has to be removed, because it's
not useful anymore.

And since the original disk def will be changed, new disk def passed
as argument will be free'ed in qemuDomainChangeEjectableMedia, so
we need to copy the orignal disk def before
qemuDomainChangeEjectableMedia, to use it for qemuRemoveSharedDisk.