qemuDomainChangeDiskLive: rework slightly

qemuDomainChangeDiskLive: rework slightly

Firstly, our coding guidelines suggest using 'cleanup' label
instead of 'end'. Then, @ret should be set to value representing
success as the last statement before the 'cleanup' label.
And while I am at this function, lets enumerate all the possible
enum items (virDomainDiskDevice) and avoid using 'default' in
switch(). Pooh. Also, nothing bad happens if we look up the disk
to change in the domain upfront. In fact, it's going to be
helpful later when we want to keep some old values for performing
a rollback.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>