print meaningful error message in case of --disable-vhost-net

When qemu gets compiled without support of vhost-net, any attempt
to use it fails with a very clear error message:

 qemu-system-x86_64: -netdev ...,vhost=on: vhost-net requested but could not be initialized

there's absolutely no reason given _why_ it coult not be
initialized, and even strace'ing the process in question
does not reveal any errors.  So print a message telling
what's going on.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
