MAINTAINERS: Update paths for AioContext implementation

Moved by c2b38b27
("block: move AioContext, QEMUTimer, main-loop to libqemuutil")

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>