qemu: Fix crash on failed VM startup

If VM startup fails early enough (can't find a referenced USB device),
libvirtd will crash trying to clear the VNC port bit, since port = 0,
which overflows us out of the bitmap bounds.

Fix this by being more defensive in the bitmap operations, and only
clearing a previously set VNC port.
Signed-off-by: NCole Robinson <crobinso@redhat.com>

src/qemu/qemu_driver.c

@@ -3773,7 +3773,7 @@ retry:
     if ((vm->def->ngraphics == 1) &&
         vm->def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_VNC &&
         vm->def->graphics[0]->data.vnc.autoport &&
-        vm->def->graphics[0]->data.vnc.port != -1) {
+        vm->def->graphics[0]->data.vnc.port >= QEMU_VNC_PORT_MIN) {
         if (virBitmapClearBit(driver->reservedVNCPorts,
                               vm->def->graphics[0]->data.vnc.port - \
                               QEMU_VNC_PORT_MIN) < 0) {

src/util/bitmap.c

@@ -119,7 +119,7 @@ int virBitmapSetBit(virBitmapPtr bitmap, size_t b)
  */
 int virBitmapClearBit(virBitmapPtr bitmap, size_t b)
 {
-    if (b > bitmap->size - 1)
+    if (bitmap->size != 0 && b > bitmap->size - 1)
         return -1;
 
     bitmap->map[VIR_BITMAP_UNIT_OFFSET(b)] &= ~(1 << VIR_BITMAP_BIT_OFFSET(b));