Allow a kernel bootconfig to set the `qemu` key

The existing code has a lot of references to the
`ro.boot.qemu` and `ro.boot.qemu.something` properties
which is not supported by the bootconfig if we place
everything under `androidboot.qemu`.

Bug: 182291166
Test: getprop | grep qemu
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: Icb9d29c8dc39e1fa52a6f2ce43b4f42182b7995d