KVM: s390: Rename some functions in gaccess.c

KVM: s390: Rename some functions in gaccess.c

Rename some functions in gaccess.c to add a _gva or _gpa suffix to
indicate whether the function accepts a virtual or a guest-absolute
address.

This makes it easier to understand the code.

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>