MAINTAINERS: Update paths for main loop

Moved by c2b38b27 ("block: move AioContext, QEMUTimer, main-loop to
libqemuutil"), let's update MAINTAINERS too.

Reported-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>