blockjob: add new --bytes flag to virsh blockjob
    
    Expose the new flag just added to virDomainGetBlockJobInfo.
    With --raw, the presence or absence of --bytes determines which
    flag to use in the single API call.  Without --raw, the use of
    --bytes forces an error if the server doesn't support it,
    otherwise, the code tries to silently fall back to scaling the
    MiB/s value.
    
    My goal is to eventually also support --bytes in bandwidth mode;
    but that's a bit further down the road (and needs a new API flag
    added in libvirt.h first).
    
    This changes the human output, but the previous patch added
    raw output precisely so that we can have flexibility with the
    human output.  For this commit, I used qemu-monitor-command to
    force an unusual bandwidth, but the same will be possible once
    qemu implements virDomainBlockCopy:
    
    Before:
    Block Copy: [100 %]    Bandwidth limit: 2 MiB/s
    After:
    Block Copy: [100 %]    Bandwidth limit: 1048577 bytes/s (1.000 MiB/s)
    
    The cache avoids having to repeatedly checking whether the flag
    works when talking to an older server, when multiple blockjob
    commands are issued during a batch session and the user is
    manually polling for job completion.
    
    * tools/virsh.h (_vshControl): Add a cache.
    * tools/virsh.c (cmdConnect, vshReconnect): Initialize the cache.
    * tools/virsh-domain.c (opts_block_job): Add --bytes.
    * tools/virsh.pod (blockjob): Document this.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>