Add a type synonym for the allocation function sig
Add a type synonym for the allocation function sig

Both iterateAlloc and tieredAlloc share the same signature, but it's
not documented nor exported (needed for refactoring).

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

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