QEMU VMs have access to host filesystem via user mode networking

[ghost]

Similar to #2385, it is possible to use user mode networking in the advanced options to access host files from a qemu VM. For details using -nic user,... / -netdev user,... see https://www.qemu.org/docs/master/system/invocation.html#hxtool-5. Problematic are the options tftp, bootfile, smb and perhaps guestfwd.

I can think of three ways to block that:

• Block the -nic and -netdev options. The normal way for configuring networking should be good enough, there should be no need to use the advanced options for that.
• Block options starting with "user," to block only user mode networking.
• Block options starting with "user," and additionally containing ",tftp=", ",bootfile=", ",smb=" or ",guestfwd=".

I would prefer the last option as this will have the least impact. Alternatively the first option is the easiest one to implement.