ppc/pnv: Introduce user creatable pnv-phb4 devices

This patch introduces pnv-phb4 user creatable devices that are created
in a similar manner as pnv-phb3 devices, allowing the user to interact
with the PHBs directly instead of creating PCI Express Controllers that
will create a certain amount of PHBs per controller index.

We accomplish this by doing the following:

- add a pnv_phb4_get_stack() helper to retrieve which stack an user
created phb4 would occupy;

- when dealing with an user created pnv-phb4 (detected by checking if
phb->stack is NULL at the start of phb4_realize()), retrieve its stack
and initialize its properties as done in stk_realize();

- use 'defaults_enabled()' in stk_realize() to avoid creating and
initializing a 'stack->phb' qdev that might be overwritten by an user
created pnv-phb4 device. This process is wrapped into a new helper
called pnv_pec_stk_default_phb_realize().

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220111131027.599784-5-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/ppc/pnv.c

@@ -1960,6 +1960,8 @@ static void pnv_machine_power9_class_init(ObjectClass *oc, void *data)
     pmc->compat = compat;
     pmc->compat_size = sizeof(compat);
     pmc->dt_power_mgt = pnv_dt_power_mgt;
+
+    machine_class_allow_dynamic_sysbus_dev(mc, TYPE_PNV_PHB4);
 }
 
 static void pnv_machine_power10_class_init(ObjectClass *oc, void *data)