s390x/css: realize css_create_sch

s390x/css: realize css_create_sch

The S390 virtual css support already has a mechanism to create a
virtual subchannel and provide it to the guest. However, to
pass-through subchannels to a guest, we need to introduce a new
mechanism to create the subchannel according to the real device
information. Thus we reconstruct css_create_virtual_sch to a new
css_create_sch function to handle all these cases and do allocation
and initialization of the subchannel according to the device type
and machine configuration.

Reviewed-by: Pierre Morel <[email protected]>
Signed-off-by: Dong Jia Shi <[email protected]>
Message-Id: <20170517004813[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>