qemu: Refactor qemuDomainObjSetJobPhase

qemu: Refactor qemuDomainObjSetJobPhase

We will want to update migration phase without affecting job ownership.
Either in the thread that already owns the job or from an event handler
which only changes the phase (of a job no-one owns) without assuming it.

Let's move the ownership change to a new qemuDomainObjStartJobPhase
helper and let qemuDomainObjSetJobPhase set the phase without touching
ownership.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>