Introduce QError

Introduce QError

QError is a high-level data type which represents an exception
in QEMU, it stores the following error information:

- class          Error class name (eg. "ServiceUnavailable")
- description    A detailed error description, which can contain
                 references to run-time error data
- filename       The file name of where the error occurred
- line number    The exact line number of the error
- function       The function name of where the error occurred
- run-time data  Any run-time error data

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>