s390/hypfs: include z/VM guests with access control group set

Currently if z/VM guest is allowed to retrieve hypervisor performance
data globally for all guests (privilege class B) the query is formed in a
way to include all guests but the group name is left empty. This leads to
that z/VM guests which have access control group set not being included
in the results (even local vm).

Change the query group identifier from empty to "any" to retrieve
information about all guests from any groups (or without a group set).

Cc: stable@vger.kernel.org
Fixes: 31cb4bd3

 ("[S390] Hypervisor filesystem (s390_hypfs) for z/VM")
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>