virtio: Extract init_vq() for PCI and MMIO Modern virtio will need to reuse this code when initializing a virtqueue. It's not much, but still nicer to have next to exit_vq(). Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com> Link: https://lore.kernel.org/r/20220701142434.75170-3-jean-philippe.brucker@arm.com Signed-off-by: Will Deacon <will@kernel.org>