Fix snabb_data_to_sysrepo data tree copy

[wungad]

When attempting to start sysrepo-snabb-plugin built for snabb-softwire-v3, an error occurs with the following message:

[INF] snabb plugin: sync sysrepo and snabb data
[INF] snabb plugin: copy snabb data to sysrepo
[INF] snabb plugin: send to socket
[INF] snabb plugin: appply snabb data to sysrepo startup datastore
[ERR] snabb plugin: failed lyd_dup_siblings_to_ctx: 5
[ERR] snabb plugin: failed to apply libyang data to sysrepo: Item not found
[ERR] snabb plugin: failed to apply snabb data to sysrepo: Item not found
[ERR] snabb plugin: failed to apply sysrepo startup data to snabb: Item not found
[ERR] sysrepo-plugind: Plugin "sysrepo-snabb-plugin" initialization failed (Item not found).

I believe the reason for this is that function lyd_dup_siblings_to_ctx tries to use non-existent schema references from target context (e.g startup) as is noted [tree_data.h]https://github.com/CESNET/libyang/blob/9a4e5b2ce30b9696116d6e654ee55caab5aafed8/src/tree_data.h#L1988-L1991)

This PR fixes this in the following way:

• data tree created in the context of libyang is first copied to sysrepo running datastore
• running datastore is copied to startup datastore.