qemu: Refresh memory size only on fresh starts

qemu: Refresh memory size only on fresh starts

Qemu unfortunately doesn't update internal state right after migration
and so the actual balloon size as returned by 'query-balloon' are
invalid for a while after the CPUs are started after migration. If we'd
refresh our internal state at this point we would report invalid current
memory size until the next balloon event would arrive.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1242940
(cherry picked from commit d7a0386e229176ec67531aac1412b8a98914da8e)