lib: add virDomainGetGuestInfo()

lib: add virDomainGetGuestInfo()

This API is intended to aggregate several guest agent information
queries and is ispired by stats API virDomainListGetStats(). It is
anticipated that this information will be provided by a guest agent
running within the domain.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>