KVM: s390: export test_vfacility()

Make test_vfacility() available for other files. This is needed for the
new guest access functions, which test if certain facilities are available
for a guest.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>