instance import: adapt to multi-disk/nic world
instance import: adapt to multi-disk/nic world

This is mostly a copy from gnt-instance add; import works, but it's not
optimal - device count/parametrs should be able to be reused.

Reviewed-by: ultrotter

ganeti-github-testing/ganeti-test-1,mbakke/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,onponomarev/ganeti,yiannist/ganeti,dimara/ganeti,andir/ganeti,grnet/snf-ganeti,leshchevds/ganeti,mbakke/ganeti,ganeti/ganeti,andir/ganeti,ganeti/ganeti,andir/ganeti,apyrgio/ganeti,leshchevds/ganeti,bitemyapp/ganeti,dimara/ganeti,yiannist/ganeti,grnet/snf-ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,ganeti/ganeti,onponomarev/ganeti,mbakke/ganeti,apyrgio/snf-ganeti