vz: fix destination domain synchronization

vz: fix destination domain synchronization

Adding domain to domain list on preparation step is not correct.
First domain is not fully constructed - domain definition is
missing. Second we can't use VIR_MIGRATE_PARAM_DEST_XML parameter
to parse definition as vz sdk can patch it by itself. Let's add/remove
domain on finish step. This is for synchronization purpose only so domain
is present/absent on destination after migration completion. Actually
domain object will probably be created right after actual vz sdk
migration start by vz sdk domain defined event.

We can not and should not sync domain cache on error path in finish step
of migration. We can not as we really don't know what is the reason of
cancelling and we should not as user should not make assumptions on
state on error path. What we should do is cleaning up temporary migration
state that is induced on prepare step but we don't have one. Thus
cancellation should be noop.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Signed-off-by: Maxim Nestratov <mnestratov@virtuozzo.com>