qemu: better error message when block job can't succeed
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1140981 reports that
    the qemu-kvm shipped as part of RHEL 7.0 intentionally[1] cripples
    block jobs by removing the 'block-stream' QMP command, while still
    leaving 'block-job-cancel' as an unusable no-op.  Meanwhile, we
    already had existing code that checked whether block jobs were
    completely missing (such as qemu 0.15), old style (cancel is
    synchronous, and all commands spelled with '_'), or new style
    (cancel is asynchronous, and all commands spelled with '-'), and
    used that three-way probe to give decent error messages.  At the
    time that code was added, all existing qemu versions fell in one
    of three buckets, and the code was using the presence of
    'block-job-cancel' as the witness of which of the three buckets.
    But now that RHEL qemu has shipped with intentionally crippled
    'block-stream', we have a fourth bucket, which results in ugly
    error messages when trying 'virsh blockpull':
    
     error: Requested operation is not valid: Command 'block-stream' is not found
    
    In reality, the fourth bucket should be treated the same as the
    first bucket (no block job support); we can do that by realizing
    that no existing build of qemu has working block-stream while
    lacking block-job-cancel, so it is easiest to change our witness
    to the command that starts a job rather than ends one.  We still
    act correctly regarding command spelling and whether cancel is
    asynchronous.  And on crippled RHEL builds, we now get the desired:
    
     error: unsupported configuration: block jobs not supported with this qemu binary
    
    [1] The intentional cripple is limited to qemu-kvm of RHEL; when using
    qemu-kvm-rhev of RHEV, block job functionality is supported.  Don't ask
    me to explain the "why" behind it all - I'm just dealing with fallout
    from someone else's decision.
    
    * src/qemu/qemu_capabilities.h (QEMU_CAPS_BLOCKJOB_SYNC): Tweak comment.
    * src/qemu/qemu_capabilities.c (virQEMUCapsCommands): Look for stream
    rather than cancel when determining the flavor of block jobs supported.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>