Makefile: Rename the 'vm-test' target as 'vm-help'

We already have 'make check-help', use the 'make vm-help' form
to display helps about VM testing. Keep the old target to not
bother old customs.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190531064341.29730-1-philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>