ignore SIGPIPE in qemu-img and qemu-io

This prevents the tools from being stopped when they write data to a
closed connection in the other side.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
