virtio: Fix vring allocation
virtio: Fix vring allocation

The value returned by virtio_vring_size() is used to allocate memory
for vring. The used descriptor list (array of vring_used_elem) is
counted by the header - vring_used struct - is not.

This fixes virtio_vring_size() to return the correct size.
At the moment rings are quite small (256) and allocated with
4096 alignment, this is why we have not been having issues with
this so far.

Reviewed-by: Thomas Huth <9e65be74847981e55b2564b6266c57d86d3bac05@linux.vnet.ibm.com>
Reviewed-by: Nikunj A Dadhania <e2f4a5db9b4735182a88051d1e7a3599f9eac9db@linux.vnet.ibm.com>
Signed-off-by: Alexey Kardashevskiy <ac1fc56530d7d037e1ea666f130daca22d72f964@ozlabs.ru>
---
Changes:
v2:
* remove magic numbers

open-power-host-os/slof,aik/SLOF,open-power-host-os/slof,stefanberger/SLOF-tpm,open-power-host-os/slof,aik/SLOF,stefanberger/SLOF-tpm,stefanberger/SLOF-tpm,qemu/SLOF,qemu/SLOF,aik/SLOF,qemu/SLOF,open-power-host-os/slof