qemu: snapshot: Terminate job when memory compression program isn't found
    
    If the compression program for external snapshot memory image isn't
    found we exitted the function without terminating the domain job. This
    caused the domain to be unusable.
    
    The problem was introduced in commit 7df5093f.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1097503