refactor: factorize SPMC's NWd VM view destruction

SPMC keeps a view of NWd VMs structure for mailbox and bitmap
handling. Factorize VM destruction code to be shared between
RXTX_UNMAP and bitmap destroy use-cases.

Change-Id: I72ebf5cfc011d5c660da244b2908142ca0568407
Signed-off-by: Federico Recanati <federico.recanati@arm.com>
diff --git a/src/api.c b/src/api.c
index 54988fc..407ca50 100644
--- a/src/api.c
+++ b/src/api.c
@@ -1419,6 +1419,7 @@
 
 	vm->mailbox.send = NULL;
 	vm->mailbox.recv = NULL;
+	plat_ffa_vm_destroy(vm_locked);
 
 	/* Forward buffer unmapping to SPMC if coming from a VM. */
 	plat_ffa_rxtx_unmap_forward(owner_vm_id);