xics: report errors with the QEMU Error API

Using the return value to report errors is error prone:
- xics_alloc() returns -1 on error but spapr_vio_busdev_realize() errors
  on 0
- xics_alloc_block() returns the unclear value of ics->offset - 1 on error
  but both rtas_ibm_change_msi() and spapr_phb_realize() error on 0

This patch adds an errp argument to xics_alloc() and xics_alloc_block() to
report errors. The return value of these functions is a valid IRQ number
if errp is NULL. It is undefined otherwise.

The corresponding error traces get promotted to error messages. Note that
the "can't allocate IRQ" error message in spapr_vio_busdev_realize() also
moves to xics_alloc(). Similar error message consolidation isn't really
applicable to xics_alloc_block() because callers have extra context (device
config address, MSI or MSIX).

This fixes the issues mentioned above.

Based on previous work from Brian W. Hart.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

trace-events

@@ -1409,8 +1409,6 @@ xics_ics_write_xive(int nr, int srcno, int server, uint8_t priority) "ics_write_
 xics_ics_reject(int nr, int srcno) "reject irq %#x [src %d]"
 xics_ics_eoi(int nr) "ics_eoi: irq %#x"
 xics_alloc(int src, int irq) "source#%d, irq %d"
-xics_alloc_failed_hint(int src, int irq) "source#%d, irq %d is already in use"
-xics_alloc_failed_no_left(int src) "source#%d, no irq left"
 xics_alloc_block(int src, int first, int num, bool lsi, int align) "source#%d, first irq %d, %d irqs, lsi=%d, alignnum %d"
 xics_ics_free(int src, int irq, int num) "Source#%d, first irq %d, %d irqs"
 xics_ics_free_warn(int src, int irq) "Source#%d, irq %d is already free"