Select "sys-usb" as the default backup location

In case there is no an existing backup profile, select sys-usb as the
default target. In case there is no `sys-usb` (e.g. on live Qubes
scenario), still revert to dom0

If dom0 is included in backup and target is user home dir in dom0,
another patch to core shows an error message. Cover it as well

resolves: QubesOS/qubes-issues#10127
depends: QubesOS/qubes-core-admin#731

Author: alimirjamali

qubesmanager/backup.py

@@ -211,6 +211,9 @@ def load_settings(self):
         try:
             profile_data = backup_utils.load_backup_profile()
         except FileNotFoundError:
+            dest_vm_idx = self.appvm_combobox.findText("sys-usb")
+            if dest_vm_idx > -1:
+                self.appvm_combobox.setCurrentIndex(dest_vm_idx)
             return
         except exc.QubesException:
             QtWidgets.QMessageBox.information(
@@ -405,6 +408,8 @@ def current_page_changed(self, page_id):  # pylint: disable=unused-argument
                 backup_summary = self.qubes_app.qubesd_call(
                     'dom0', 'admin.backup.Info',
                     backup_utils.get_profile_name(True)).decode()
+            except exc.QubesException as e:
+                backup_summary = str(e)
             except exc.QubesDaemonAccessError:
                 backup_summary = "Failed to get backup summary: " \
                                  "insufficient permissions"