SSHFS process channel leaks

[ricab]

Describe the bug
The SFTP session releases the ssh_channel on the SSHFS process, but it never frees it. Ownership is transferred but not properly adopted. Libssh's sftp_server_new doesn't transfer ownership and sftp_server_free does not free the channel.

multipass/src/sshfs_mount/sftp_server.cpp

Line 313 in f4542c3

sftp_server_session{make_sftp_session(ssh_session, sshfs_process->release_channel())},

multipass/src/sshfs_mount/sftp_server.cpp

Lines 65 to 66 in f4542c3

	mp::SftpServer::SftpSessionUptr sftp_server_session{sftp_server_new(session, channel),
	sftp_server_free};