server tests: Update the test NAME of multihost_migration_mix

The original NAME for this test is duplicated with multihost_migration.
And this will cause problem when we install the autotest server.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #13 from ypu/multihost_migration_mix_name

server tests: Update the test NAME of multihost_migration_mix