qemu.tests.cfg: Disable migrate.unix on RHEL5 host

migrate.unix protocol only support from RHEL6, so disable it on RHEL5 host.

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #60 from spcui/migrate_fix

qemu.tests.cfg: Disable migrate.unix on RHEL5 host