qemuimagetest: Use the same image in sanity testing to fix the timeout issue on autobuilder
Fixes [BUGID #595]
Because of the QEMU booting slowness issue(see bug #646 and #618), autobuilder
may suffer a timeout issue when running sanity test. We introduce variable
SHARE_IMAGE here to fix the issue. It is by default set to 1. Poky will copy
latest built-out image and keep using it in sanity testing. If it is set to 0,
latest built-out image will be copied and tested for each case, which will take
much time.
