migration: merge all proto branches into single function

migration: merge all proto branches into single function

Finally on this step we get what we were aimed for - toURI{1, 2} (and
migration{*} APIs too) now can work thru V3_PARAMS protocol. Execution path
goes thru unchanged virDomainMigrateUnmanaged adapter function which is called
by all target places.

Note that we keep the fact that direct migration never works
thru V3_PARAMS proto. We can't change this aspect without
further investigation.

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