qemu: domain: Store data for 'secret' object representing http cookies

qemu: domain: Store data for 'secret' object representing http cookies

The http cookies can have potentially sensitive values and thus should
not be leaked into the command line. This means that we'll need to
instantiate a 'secret' object in qemu to pass the value encrypted.

This patch adds infrastructure for storing of the alias in the status
XML.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>