qga: Shorten several error messages

qga: Shorten several error messages

Some, but not all error messages are of the form

    Guest agent command failed, error was '<actual error message>'

For instance, command guest-exec can fail with an error message like

    Guest agent command failed, error was 'Failed to execute child process “/bin/invalid-cmd42” (No such file or directory)'

Shorten this to just just the actual error message.  The guest-exec
example becomes

    Failed to execute child process “/bin/invalid-cmd42” (No such file or directory)

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240514105829.729342-3-armbru@redhat.com>
[Superfluous #include "qapi/qmp/qerror.h" deleted]
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>