Comment # 1 on bug 981836 from Charles Arnold

Virt-manager on aarch64 does the following by design.

"
If we detect AAVMF, set it up, and allow all install options.
If we don't find AAVMF, disable all options except import install (the
current behavior) and show a warning.
"

Because it is not finding AAVMF only import is enabled.