launch: board model for RISC-V

launch: board model for RISC-V

On RISC-V there is no default machine type. Invoking QEMU requires to
specify a board model with the -M option. So let's define MACHINE_TYPE
as virt.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Gbp-Pq: Name 0022-launch-board-model-for-RISC-V.patch