bug fix: Set full path to guest page file in guest tab

Commit 6bcbb3f0 changed how the guest tab is loaded to be more robust, but it
was using relative path to guest page which causes errors when Kimchi is
running outside its top directory.
So use full path to avoid those kind of problems.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>