bhyve: implement virDomainAgentSetResponseTimeout()

bhyve: implement virDomainAgentSetResponseTimeout()

Implement the virDomainAgentSetResponseTimeout() which allows to
set the qemu guest agent timeout.

The change consists of two main parts:

 - bhyveDomainAgentSetResponseTimeout() driver method implementation
   which actually sets the agent timeout.
 - Updating virDomainXMLPrivateDataCallbacks with "parse" and "format"
   methods implementation for parsing and formatting of the agent
   timeout value in domain's XML.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>