Return -1 in virPortAllocatorAcquire if all ports are used

Report the error in virPortAllocatorAcquire instead
of doing it in every caller.

The error contains the port range name instead of the intended
use for the port, e.g.:
Unable to find an unused port in range 'display' (65534-65535)
instead of:
Unable to find an unused port for SPICE

This also adds error reporting when the QEMU driver could not
find an unused port for VNC, VNC WebSockets or NBD migration.

tests/virportallocatortest.c

@@ -118,11 +118,9 @@ static int testAllocAll(const void *args ATTRIBUTE_UNUSED)
         goto cleanup;
     }
 
-    if (virPortAllocatorAcquire(alloc, &p7) < 0)
-        goto cleanup;
-    if (p7 != 0) {
+    if (virPortAllocatorAcquire(alloc, &p7) == 0) {
         if (virTestGetDebug())
-            fprintf(stderr, "Expected 0, got %d", p7);
+            fprintf(stderr, "Expected error, got %d", p7);
         goto cleanup;
     }