tests: add (riscv virt) machine mapping to testenv

tests: add (riscv virt) machine mapping to testenv

Some qemu-iotests(040 etc) use PCI disk to do test. Without the
mapping, RISC-V flavor use spike as default machine which has no
PCI bus, causing test failure.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/894

Signed-off-by: Kai Zhang <laokz@foxmail.com>
Message-Id: <tencent_E4219E870165A978DB5BBE50BD53D33D2E06@qq.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>