src: add constants for guest info 'load.' parameters

src: add constants for guest info 'load.' parameters

Contrary to most APIs returning typed parameters, there are no constants
defined for the guest info data keys. This is was because many of the
keys needs to be dynamically constructed using one or more array index
values.

It is possible to define constants while still supporting dynamic
array indexes by simply defining the prefixes and suffixes as constants.
The consuming code can then combine the constants with array index
value.

With this approach, it is practical to add constants for the guest info
API keys.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>