KVM: return an error code in kvm_vm_ioctl_register_coalesced_mmio()

KVM: return an error code in kvm_vm_ioctl_register_coalesced_mmio()

If kvm_io_bus_register_dev() fails then it returns success but it should
return an error code.

I also did a little cleanup like removing an impossible NULL test.

Cc: stable@vger.kernel.org
Fixes: 2b3c246a682c ('KVM: Make coalesced mmio use a device per zone')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit aac5c4226e7136c331ed384c25d5560204da10a0)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>