ui: guest import: use box-label style for prepare-for-virtio checkbox

ui: guest import: use box-label style for prepare-for-virtio checkbox

Aaron originally designed the new "unique MAC address" checkbox after
this one for alignment reasons [0]. I changed it and argued that's
fine if there is a misalignment between those components [1], which I
still think, but tbh. I also like the boxLabel more for the "Prepare
for VirtIO" checkbox, so lets use it there too and hope all are happy
again.

[0]: https://lists.proxmox.com/pipermail/pve-devel/2024-March/062411.html
[1]: https://lists.proxmox.com/pipermail/pve-devel/2024-March/062412.html

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>