rpc: for messages with FDs always decode count of FDs from the message
    
    The packet with passed FD has the following format:
    
        --------------------------
        | len | header | payload |
        --------------------------
    
    where "payload" has an additional count of FDs before the actual data:
    
        ------------------
        | nfds | payload |
        ------------------
    
    When the packet is received we parse the "header", which as a side
    effect updates msg->bufferOffset to point to the beginning of "payload".
    If the message call contains FDs, we need to also parse the count of
    FDs, which also updates the msg->bufferOffset.
    
    The issue here is that when we attempt to read the FDs data from the
    socket and we receive EAGAIN we finish the reading and call poll()
    to wait for the data the we need.  When the data arrives we already have
    the packet in our buffer so we read the "header" again but this time
    we don't read the count of FDs because we already have it stored.
    
    That means that the msg->bufferOffset is not updated to point to the
    actual beginning of the payload data, but it points to the count of
    FDs.  After all FDs are processed we dispatch the message to process
    it and decode the payload.  Since the msg->bufferOffset points to wrong
    data, we decode the wrong payload and the API call fails with
    error messages:
    
        Domain not found: no domain with matching uuid '67656e65-7269-6300-0c87-5003ca6941f2' ()
    
    Broken by commit 133c511b527 which fixed a FD and memory leak.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>