Add a helper for the "iallocator" opcode field
Add a helper for the "iallocator" opcode field

This field is used with just changed description in about 10 opcodes,
so unifying it makes things simpler for future potential changes to
the field type.

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

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