qapi: Add human mode to StringOutputVisitor

qapi: Add human mode to StringOutputVisitor

This will be used by "info qtree".  For numbers it prints both the
decimal and hex values.  For sizes it rounds to the nearest power
of 2^10.  For strings, it puts quotes around the string and separates
NULL and empty string.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>