parse_keyval: Allow hex vals

When parse_keyval was first written we didn't yet have strtol.
Now we do, let's give users more flexibility.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20211008070309.84205-1-drjones@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>