vhost-user-test: small changes to init_hugepagefs

After the conversion to qgraph, the equivalent of "main" will be in
a constructor and will run even if the tests are not being requested.
Therefore, it should not assert that init_hugepagefs succeeds and will
be called when creating the TestServer.  This patch changes the prototype
of init_hugepagefs, this way the next patch looks nicer.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1543851204-41186-14-git-send-email-pbonzini@redhat.com>
Message-Id: <1550165756-21617-9-git-send-email-pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>