server.tests: Add a server case for multihost migration

Add a case to run multihost migration with other cases in virt-test
from tp-qemu together. Some vaule can be set based on your owner
env:
REPEAET_NUM: How many cases you want to have in your test loop
CASES: Test cases that you want to run in your test loop
FIXED_CASES: You can add the cases that you want to run more than once to this list.
             It can guarantee the test cases are tested enough in the loop.
PREPARE_CASES: Some cases like unattended_install to prepare the env or images. This
               will not be counted to repeat times.

This will help to run some more complex scenarios in our tests by reusing the exist
test cases.