d/tests/control: use qemu-system-riscv64 (virtual) package in Depends instead of qemu-system-misc

qemu-system-foo packages are groups of architectures which can be rearranged, by splitting some architecture groups into their own packages or merging them into larger groups. But qemu-system packages provide virtual packages named after binaries they ship. Use this mechanism for riscv64 binary in tests to allow qemu-system-misc => qemu-system-riscv package rearrangement