qemu_migration: Don't set unlimited memlock limit for RDMA

qemu_migration: Don't set unlimited memlock limit for RDMA

Our documentation says RDMA migration requires hard_limit to be set so
that we know how big memory locking limit should be set for the domain
during migration. But since commit v1.2.13-71-gcf521fc8ba (which changed
the default hard_limit value from 0 to
VIR_DOMAIN_MEMORY_PARAM_UNLIMITED) we were actually setting memlock
limit to unlimited if hard_limit was not set.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>