Fix possible memory leak in util/virxml.c
    
    A "xmlstr" string may not be assigned into a "doc" pointer and it
    could cause memory leak. To fix it if the "doc" pointer is NULL and
    the "xmlstr" string is not assigned we should free it.
    
    This has been found by coverity.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>