s390x/css: realize css_sch_build_schib

s390x/css: realize css_sch_build_schib

The S390 virtual css support already has a mechanism to build a
virtual subchannel information block (schib) and provide virtual
subchannels to the guest. However, to pass-through subchannels to
a guest, we need to introduce a new mechanism to build its schib
according to the real device information. Thus we realize a new css
sch_build_schib function to extract the path_masks, chpids, chpid
type from sysfs. To reuse the existing code, we refactor
css_add_virtual_chpid to css_add_chpid.

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