KVM: s390: vsie: Allow non-zarch guests

KVM: s390: vsie: Allow non-zarch guests

Linux/KVM runs in z/Architecture-only mode. Although z/Architecture
is built upon a long history of hardware refinements, any other
CPU mode is not permitted.

Allow a userspace to explicitly enable the use of ESA mode for
nested guests, otherwise usage will be rejected.

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>