conf: report errors when parsing video acceleration

conf: report errors when parsing video acceleration

Since this function is now only called when an 'acceleration' element is
present in the xml, any failure to parse the element will be considered
an error.

Previously, we detected some types of errors, but we would only log an
error (virReportError()), but still return a partially-specified accel
object to the caller. This patch returns NULL for all parsing errors and
reports that error back up to the caller.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>