kraft.yaml: Update platform name and rootfs path

Update platform name from `kvm` to `qemu`, since the default naming
convention was changed in release 0.13.0.
Also update the rootfs name, from `./minrootfs.tgz` to `./rootfs.tar.gz`.

Signed-off-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #12

kraft.yaml

@@ -14,9 +14,9 @@ unikraft:
     - CONFIG_LIBUKLIBPARAM=y
 targets:
   - architecture: x86_64
-    platform: kvm
+    platform: qemu
   - architecture: arm64
-    platform: kvm
+    platform: qemu
 libraries:
   musl:
     version: stable
@@ -39,4 +39,4 @@ libraries:
 volumes:
   fs0:
     driver: 9pfs
-    source: ./minrootfs.tgz
+    source: ./rootfs.tar.gz