vmspawn: use our generic vsock CID parser, instead of a howngrown one

Let's also use 'unsigned' as type for the CID everywhere, and
VMADDR_CID_ANY as marker for "invalid CID", as that's what the vsock
APIs natively do.

src/basic/socket-util.h

@@ -374,6 +374,11 @@ int socket_get_mtu(int fd, int af, size_t *ret);
 
 int connect_unix_path(int fd, int dir_fd, const char *path);
 
+static inline bool VSOCK_CID_IS_REGULAR(unsigned cid) {
+        /* 0, 1, 2, UINT32_MAX are special, refuse those */
+        return cid > 2 && cid < UINT32_MAX;
+}
+
 int vsock_parse_port(const char *s, unsigned *ret);
 int vsock_parse_cid(const char *s, unsigned *ret);