qemu: Limit rtc-reset-reinjection requirement to x86 only.
    
    The QMP command, like the interrupt reinjection logic it's connected
    to, is only implemented in QEMU when TARGET_I386 is defined, so
    checking for its availability on any other architecture is pointless.
    
    On the other hand, when we're on x86, we shouldn still make sure that
    rtc-reset-reinjection is available and refuse to set the time
    otherwise.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1211938