virtio-blk: allow toggling host cache between writeback and writethrough

This patch adds support for the new VIRTIO_BLK_F_CONFIG_WCE feature,
which exposes the cache mode in the configuration space and lets the
driver modify it.  The cache mode is exposed via sysfs.

Even if the host does not support the new feature, the cache mode is
visible (thanks to the existing VIRTIO_BLK_F_WCE), but not modifiable.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>