virt: Verify whether guest image is bootable

This patch adds a function in vm.verify_alive method,
to check if the image is bootable. If image can't bootup,
autotest will raise an ImageUnbootableError exception.

There are 2 parameters for this function:
 * image_verify_bootable: Turn on/off this checking function.
 * image_unbootable_pattern: Pattern for bootable checking.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>