The mode attribute is required for the source element of vhost-user.
Thus virDomainNetDefFormat should always generate a xml with it and not
only when the mode is server.

The commit fixes the issue. And it adds a vhostuser interface in
'client' mode to qemuxml2argv-net-vhostuser.(args|xml) to test this
usecase.
Signed-off-by: NMaxime Leroy <maxime.leroy@6wind.com>