[electrum] fix x_pubkey format in multisig wallet

Summary: For now the wallet still works with coin dicts containing hex strings. D14594 converted keystore.get_xpubkey to return bytes, and applied the correct hex conversion in SimpleDeterministicWallet.add_input_sig_info, but missed the conversion in MultisigWallet.add_input_sig_info

Test Plan:
python test_runner.py

Send a transaction with a multisig setup

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14605