[SkyServe] Check positive for target_qps_per_replica

[cblmemo]

Our autoscaling metric target_qps_per_replica should be a positive number. This PR adds a check on the constraint.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)

# neg-tar-qps.yaml
service:
  readiness_probe: /
  replica_policy:
    min_replicas: 2
    max_replicas: 10
    target_qps_per_replica: -2.5 # also tested with 0

resources:
  ports: 5000

$ sky serve up neg-tar-qps.yaml
Service from YAML spec: neg-tar-qps.yaml
ValueError: target_qps_per_replica must be greater than 0

• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[MaoZiming]

LGTM