s390/hypfs: fix a typo in the name of a function

Everything is about hypfs_..., except 'hpyfs_vm_create_guest()'
s/hpy/hyp/

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>