Fix qemu-guest-agent startup failure by replacing --blacklist with --block-rpcs

[Travisivart]

Merge Checklist

• The toolchain has been rebuilt successfully (or no changes were made to it)
• The toolchain/worker package manifests are up-to-date
• Any updated packages successfully build (or no packages were changed)
• Packages depending on static components modified in this PR (Golang, *-static subpackages, etc.) have had their Release tag incremented.
• Package tests (%check section) have been verified with RUN_CHECK=y for existing SPEC files, or added to new SPEC files
• All package sources are available
• cgmanifest files are up-to-date and sorted
• LICENSE-MAP files are up-to-date
• All source files have up-to-date hashes in the *.signatures.json files
• sudo make go-tidy-all and sudo make go-test-coverage pass
• Documentation has been updated to match any changes to the build system
• Ready to merge

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Summary

QEMU 9.1 removed the deprecated --blacklist CLI option from qemu-ga, replacing it with --block-rpcs. The qemu-guest-agent systemd service file and sysconfig still referenced --blacklist, causing the guest agent to fail to start with:

qemu-ga: unrecognized option '--blacklist'

This PR updates the service file and sysconfig to use the new --block-rpcs flag and renames the BLACKLIST_RPC environment variable to BLOCK_RPCS.

Change Log

• Replace --blacklist=${BLACKLIST_RPC} with --block-rpcs=${BLOCK_RPCS} in qemu-guest-agent.service
• Rename BLACKLIST_RPC to BLOCK_RPCS and update comments in qemu-ga.sysconfig
• Bump qemu Release from 1 to 2 and add changelog entry

Does this affect the toolchain?

NO

Associated issues

• Fixes Latest qemu-guest-agent 9.1 RPM does not work due to change in expected paremeters #15843

Test Methodology

• Verified qemu-ga --help confirms --block-rpcs is the correct replacement flag
• Confirmed via upstream QEMU commit and Red Hat BZ#2258100 that --blacklist was removed in QEMU 9.1
• No source tarballs or signatures changed (only service/sysconfig files and spec release bump)