s390x/css: add s390-squash-mcss machine option

We want to support real (i.e. not virtual) channel devices
even for guests that do not support MCSS-E (where guests may
see devices from any channel subsystem image at once). As all
virtio-ccw devices are in css 0xfe (and show up in the default
css 0 for guests not activating MCSS-E), we need an option to
squash both the virtio subchannels and e.g. passed-through
subchannels from their real css (0-3, or 0 for hosts not
activating MCSS-E) into the default css. This will be
exploited in a later patch.

Signed-off-by: Xiao Feng Ren <renxiaof@linux.vnet.ibm.com>
Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Message-Id: <20170517004813.58227-4-bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
