s390x: Test for share/unshare call support before using them

Testing for facility only means the UV Call facility is available.
The UV will only indicate the share/unshare calls for a protected
guest 2, so lets also check that.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>