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: NQingtang Zhou <qzhou@redhat.com>

tests/cfg/pxe.cfg

@@ -12,3 +12,4 @@
     network = bridge
     restart_vm = yes
     pxe_timeout = 60
+    image_verify_bootable = no

tests/cfg/unattended_install.cfg

@@ -17,6 +17,7 @@
     inactivity_treshold = 1800
     # Set migrate_background to yes to run migration in parallel
     # migrate_background = yes
+    image_verify_bootable = no
 
     # Way of delivering ks file into the guest
     variants: