fix: sev_get_capabilities call get sev-device object with no SEV configured

[wdsun1008]

No description provided.

[github-actions]

Thank you for your interest in the QEMU project.

This repository is a read-only mirror of the project's repostories hosted
on https://gitlab.com/qemu-project/qemu.git.
The project does not process merge requests filed on GitHub.

QEMU welcomes contributions of code (either fixing bugs or adding new
functionality). However, we get a lot of patches, and so we have some
guidelines about contributing on the project website:
https://www.qemu.org/contribute/