AArch64 system emulation cannot execute virt uefi in 2.7 or 2.8

The UEFI firmware file is retrieved from http://snapshots.linaro.org/components/kernel/linaro-edk2/latest/release/qemu64/QEMU_EFI.fd .

The error is:
```
TODO /var/lib/abbs/build/tmp.p2dMBBlJ0D/qemu-2.7.0/tci.c:1049: tcg_qemu_tb_exec()
/var/lib/abbs/build/tmp.p2dMBBlJ0D/qemu-2.7.0/tci.c:1049: tcg fatal error
```

(both 2.7.0 and 2.8.0 are tested to fail, 2.6.1 works)