configure: Provide and use convenience error reporting function

Provide a convenience function for reporting an error and exiting,
and update various places in the configure script to use it.
This allows us to be a little more consistent about how format
our error messages and makes the calling code shorter.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1365419487-19867-2-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>