This CL includes x86_64 specific changes that are required to run
microdroid on cuttlefish_x86_64.
1) Unsigned bootloader is used. (b/185115783)
2) androidboot.boot_devices is correctly set for the architecture
3) uboot-env.txt is customized to set loadaddr
4) MicrodroidTestCase is revised to handle the case when multiple
devices are connected.
Bug: 185115783
Bug: 185082754
Test: atest MicrodroidHostTests with aosp_cf_x86_64_phone
Change-Id: I1baebedd392f742f9a9cd064d63a8c3741e46daa