fakedbapi.cpv_exists: handle KeyError from _instance_key

Since commit 7921e61065502fd0bb08d9dfef6a4493657961bf, cpv_exists
needs to handle KeyError from _instance_key_multi_instance.

Fixes: 7921e6106550 ("binpkg-multi-instance 2 of 7")
diff --git a/pym/portage/dbapi/virtual.py b/pym/portage/dbapi/virtual.py
index a59c3b5..f2e841f 100644
--- a/pym/portage/dbapi/virtual.py
+++ b/pym/portage/dbapi/virtual.py
@@ -98,8 +98,12 @@
 		return result[:]
 
 	def cpv_exists(self, mycpv, myrepo=None):
-		return self._instance_key(mycpv,
-			support_string=True) in self.cpvdict
+		try:
+			return self._instance_key(mycpv,
+				support_string=True) in self.cpvdict
+		except KeyError:
+			# _instance_key failure
+			return False
 
 	def cp_list(self, mycp, use_cache=1, myrepo=None):
 		# NOTE: Cache can be safely shared with the match cache, since the