nbd: allow setting of an export name for qemu-nbd server

nbd: allow setting of an export name for qemu-nbd server

The qemu-nbd server currently always uses the old style protocol
since it never sets any export name. This is a problem because
future TLS support will require use of the new style protocol
negotiation.

This adds "--exportname NAME" / "-x NAME" arguments to qemu-nbd
which allow the user to set an explicit export name. When an
export name is set the server will always use the new style
NBD protocol.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1455129674-17255-11-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>