hwrng: virtio - don't waste entropy

if we don't use all the entropy available in the buffer, keep it
and use it later.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Link: https://lore.kernel.org/r/20211028101111.128049-4-lvivier@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>