mom: Fix KeyError exception while qemu-guest-agent error handling

Current version of qemu-guest-agent reports error messages in the format:
    {"error":{"class":"GenericError","desc":"..."}}

GuestQemuAgent tries to get "data" member on the error response, and results
in KeyError exception as the response doesn't have it. Catch the exception,
and also reference "desc" member for the error message.

Change-Id: Ibfb84eac189716513f686d5b035056a0c1bc36d3
Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-on: http://gerrit.ovirt.org/18887
Reviewed-by: Adam Litke <agl@us.ibm.com>
Tested-by: Adam Litke <agl@us.ibm.com>