libvhost-user: prevent over-running max RAM slots

When VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS support was added to
libvhost-user, no guardrails were added to protect against QEMU
attempting to hot-add too many RAM slots to a VM with a libvhost-user
based backed attached.

This change adds the missing error handling by introducing a check on
the number of RAM slots the device has available before proceeding to
process the VHOST_USER_ADD_MEM_REG message.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20220117041050.19718-6-raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

subprojects/libvhost-user/libvhost-user.c

@@ -705,6 +705,14 @@ vu_add_mem_reg(VuDev *dev, VhostUserMsg *vmsg) {
         return false;
     }
 
+    if (dev->nregions == VHOST_USER_MAX_RAM_SLOTS) {
+        close(vmsg->fds[0]);
+        vu_panic(dev, "failing attempt to hot add memory via "
+                      "VHOST_USER_ADD_MEM_REG message because the backend has "
+                      "no free ram slots available");
+        return false;
+    }
+
     /*
      * If we are in postcopy mode and we receive a u64 payload with a 0 value
      * we know all the postcopy client bases have been received, and we