Fix problem that build_target of instance name is always none.

Bug: 117590969
Test: m acloud && atest acloud_test &&
run $acloud create and check instance name should be like
"ins-454badf2-5062173-aosp-cf-x86-phone-userdebug"

Change-Id: I69ca02d8c2c9b77b123d82864b36889407e6dd80
1 file changed