[RFE] option to suppress gemu_log() output

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1828723
Reported by 'Thomas Moschny' on 2019-05-12 :

With user mode emulation, messages from genu_log() are emitted
unconditionally to stderr. I didn't find a way to suppress them. It would
be nice to have options similar to the -D/-d options to be able to filter
and/or redirect the output.

My use case is chroot/container execution for different architectures via
binfmt. In this case it will appear as if the binary in question had
emitted messages like "Unsupported setsockopt ..." to stderr while in fact
the message came from qemu-*-static.