KVM: s390: Manually invoke vcpu setup during kvm_arch_vcpu_create()

Rename kvm_arch_vcpu_setup() to kvm_s390_vcpu_setup() and manually call
the new function during kvm_arch_vcpu_create().  Define an empty
kvm_arch_vcpu_setup() as it's still required for compilation.  This
is effectively a nop as kvm_arch_vcpu_create() and kvm_arch_vcpu_setup()
are called back-to-back by common KVM code.  Obsoleting
kvm_arch_vcpu_setup() paves the way for its removal.

Note, gmap_remove() is now called if setup fails, as s390 was previously
freeing it via kvm_arch_vcpu_destroy(), which is called by common KVM
code if kvm_arch_vcpu_setup() fails.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>