fix(VastAICompute): filter region before offer (#3193)

* feat(VastAICompute): add support for region

* fix(VastAICompute): use iso country code in offer

* Revert "fix(VastAICompute): use iso country code in offer"

This reverts commit 0ba98a2.

Author: DragonStuff

src/dstack/_internal/core/backends/vastai/compute.py

@@ -58,6 +58,7 @@ def get_offers_by_requirements(
     ) -> List[InstanceOfferWithAvailability]:
         offers = get_catalog_offers(
             backend=BackendType.VASTAI,
+            locations=self.config.regions or None,
             requirements=requirements,
             # TODO(egor-s): spots currently not supported
             extra_filter=lambda offer: not offer.instance.resources.spot,