qemu-img: Check getchar() return value in read_password() for WIN32

getchar() is a standard c library function which may return with failure
(e.g. -1), so like another platforms, also need check it under WIN32.

And make the related code match current qemu code styles, too.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
