run: Make safe behavior the default, change options

In order to save time in virt-test:

1) The JeOS image is uncompressed once and reused
   every test, unless it is explicitly asked with
   --restore-image

2) After a test is done, the VMs won't be shut down
   unless it is explicitly asked with --restart-vm

3) After a test is done, the pristine image won't
   be restored, unless it is explicitly asked with
   --restore-image-between-tests

All of those behaviors, while might bring speed ups
to the test process, they are not safe. As users
frequently don't have the visibility of that when
using the test suite, we found out that it's in the
best interest of users and developers that the
defaults are changed to safer options.

With this, the options mentioned above changed to

--keep-guest-running
--keep-image
--keep-image-between-tests

The wording 'guest' was chose over 'vm' due to the
fact we already have --guest-os. So, mostly an attempt
to keep consistency.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>