emulators/qemu-guest-agent: fixes disabled-rpcs broke service start

[jkellerer]

Applies renamed CLI arg blacklist > block-rpcs.
See: https://bugzilla.redhat.com/show_bug.cgi?id=2156515

qemu-ga --help | grep -E 'black|block' -C5
  -t, --statedir    specify dir to store state information (absolute paths
                    only, default is /var/run)
  -v, --verbose     log extra debugging information
  -V, --version     print version information and exit
  -d, --daemonize   become a daemon
  -b, --block-rpcs  comma-separated list of RPCs to disable (no spaces,
                    use "--block-rpcs=help" to list available RPCs)
  -a, --allow-rpcs  comma-separated list of RPCs to enable (no spaces,
                    use "--allow-rpcs=help" to list available RPCs)
  -D, --dump-conf   dump a qemu-ga config file based on current config
                    options / command-line parameters to stdout
  -r, --retry-path  attempt re-opening path if it's unavailable or closed

May also fix #4255

[fraenki]

Merged, thanks!