tests/hmp: Fix typo in the 'chardev-send-break' test

testchardev2 is not a valid chardev id here. Use testchardev1
instead which has been created with chardev-add right before
the 'chardev-send-break' line.
And while we're at it, add the test-hmp.c file to the MAINTAINERS
file, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1501149097-19071-1-git-send-email-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>