virtio-net: split the has_buffers() logic from can_receive()

We should only return zero from receive() for a condition which we'll
get notification of when it changes. Currently, we're returning zero
if the guest driver is not ready, but we won't ever flush our queue
when that status changes.

Also, don't check buffer space in can_receive(), but instead just allow
receive() to return zero when this condition occurs and have the caller
handle queueing the packet.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
