qemu: capabilities: Remove support for downstream-only QMP monitor backport
    
    Some distros (see diff) chose to backport QMP support rather than rebase
    to newer version of qemu. As a hack they added the string 'libvirt' to
    the qemu -help output. Remove this as downstream-only hacks should be
    carried by downstream and not litter upstream.
    
    This effectively reverts commit ff88cd590572277f10ecee4ebb1174d9b70fc0d7