s390/uv: fix prot virt host indication compilation

prot_virt_host is only available if CONFIG_KVM is enabled. So lets use
a variable initialized to zero and overwrite it when that config
option is set with prot_virt_host.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Fixes: 37564ed8 ("s390/uv: add prot virt guest/host indication files")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>