migration: refactor: introduce parameter checking function

migration: refactor: introduce parameter checking function

virDomainMigrateUnmanagedParams is not a good candidate for this functionality
as it is used by migrate family functions too and its have its own checks that
are superset of extracted and we don't need to check twice.

Actually name of the function is slightly misleading as there is also a check
for consistensy of flags parameter alone. So it could be refactored further and
reused by all migrate functions but for now let it be a matter of a different
patchset.

It is *not* a pure refactoring patch as it introduces offline check for older
versions. Looks like it must be done that way and no one will be broken too.

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