Create is_pairable() function and use it.

Fixes #1653.

lib/plugins.py

@@ -376,7 +376,7 @@ class DeviceMgr(PrintError):
             info = self.select_device(wallet, plugin, devices)
             if info:
                 client = self.client_lookup(info.device.id_)
-                if client and not client.features.bootloader_mode:
+                if client and client.is_pairable():
                     # See comment above for same code
                     client.handler = wallet.handler
                     # This will trigger a PIN/passphrase entry request

plugins/trezor/clientbase.py

@@ -94,6 +94,9 @@ class TrezorClientBase(GuiMixin, PrintError):
         '''True if initialized, False if wiped.'''
         return self.features.initialized
 
+    def is_pairable(self):
+        return not client.features.bootloader_mode
+
     @staticmethod
     def expand_path(n):
         '''Convert bip32 path to list of uint32 integers with prime flags