MAINTAINERS: use 'https://' instead of 'git://' for GitHub

MAINTAINERS: use 'https://' instead of 'git://' for GitHub

When you clone the repository without previous commit history, 'git://'
doesn't protect from man-in-the-middle attacks.  HTTPS is more secure
since the client verifies the server certificate.

Reported-by: Jann Horn <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Acked-by: Cornelia Huck <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20181108111531[email protected]
Signed-off-by: Peter Maydell <[email protected]>