lxc: Fix virLXCDomainObjBeginJob position in lxcDomainSetMemoryParameters
    
    Adjust the code to perform the virLXCDomainObjBeginJob first
    and then the call virDomainLiveConfigHelperMethod.
    As Ján Tomko pointed out, in virDomainLiveConfigHelperMethod,
    there is a check to see if the domain is active when AFFECT_LIVE is set.
    Since virLXCDomainObjBeginJob unlocks the virDomainObjPtr lock,
    the domain could possibly be destroyed while we wait for the job
    and the check results would no longer be valid.
    
    Signed-off-by: Katerina Koukiou <k.koukiou@gmail.com>