s390x/kvm: Get rid of legacy_s390_alloc()

s390x/kvm: Get rid of legacy_s390_alloc()

legacy_s390_alloc() was required for dealing with the absence of the ESOP
feature -- on old HW (< gen 10) and old z/VM versions (< 6.3).

As z/VM v6.2 (and even v6.3) is no longer supported since 2017 [1]
and we don't expect to have real users on such old hardware, let's drop
legacy_s390_alloc().

Still check+report an error just in case someone still runs on
such old z/VM environments, or someone runs under weird nested KVM
setups (where we can manually disable ESOP via the CPU model).

No need to check for KVM_CAP_GMAP - that should always be around on
kernels that also have KVM_CAP_DEVICE_CTRL (>= v3.15).

[1] https://www.ibm.com/support/lifecycle/search?q=z%2FVM

Suggested-by: Cornelia Huck <cohuck@redhat.com>
Suggested-by: Thomas Huth <thuth@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Halil Pasic <pasic@linux.ibm.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20210303130916.22553-2-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>