Frontends can trigger OOM in Backends by update a watched path

ISSUE DESCRIPTION

Some OSes (such as Linux, FreeBSD, NetBSD) are processing watch events using a single thread. If the events are received faster than the thread is able to handle, they will get queued.
As the queue is unbound, a guest may be able to trigger a OOM in the backend.

IMPACT

A malicious guest can trigger an OOM in backends.

VULNERABLE SYSTEMS

All systems with a FreeBSD, Linux, NetBSD dom0 are vulnerable.
All version of those OSes are vulnerable.