spapr_pci: fix irq leak in RTAS ibm,change-msi

This RTAS call is used to request new interrupts or to free all interrupts.

If the driver has already allocated interrupts and asks again for a non-null
number of irqs, then the rtas_ibm_change_msi() function will silently leak
the previous interrupts.

It happens because xics_free() is only called when the driver releases all
interrupts (!req_num case). Note that the previously allocated spapr_pci_msi
is not leaked because the GHashTable is created with destroy functions and
g_hash_table_insert() hence frees the old value.

This patch makes sure any previously allocated MSIs are released when a
new allocation succeeds.

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