idpf: change the method for mailbox workqueue allocation

Since workqueues are created per CPU, the works scheduled to this
workqueues are run on the CPU they were assigned. It may result in
overloaded CPU that is not able to handle virtchnl messages in
relatively short time. Allocating workqueue with WQ_UNBOUND and
WQ_HIGHPRI flags allows scheduler to queue virtchl messages on less loaded
CPUs, what eliminates delays.

Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Milena Olech <milena.olech@intel.com>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>