KVM: remove unneeded return value of vcpu_postcreate

The return value of kvm_arch_vcpu_postcreate is not checked in its
caller.  This is okay, because only x86 provides vcpu_postcreate right
now and it could only fail if vcpu_load failed.  But that is not
possible during KVM_CREATE_VCPU (kvm_arch_vcpu_load is void, too), so
just get rid of the unchecked return value.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>