migration: refactor: merge direct and p2p into unmanaged

migration: refactor: merge direct and p2p into unmanaged

p2p plain and direct function are good candidates for code reuse. Their main
function is same - to branch among different versions of migration protocol and
implementation of this function is also same. Also they have other common
functionality in lesser aspects. So let's merge them.

But as they have different signatures we have to get to convention on how to
pass direct migration 'uri' in 'dconnuri' and 'miguri'. Fortunately we alreay
have such convention in parameters passed to toURI2 function, just let's follow
it. 'uri' is passed in miguri and dconnuri is ignored.

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