spapr: fix spapr-nvram migration

spapr: fix spapr-nvram migration

When spapr-nvram is backed by a file using pflash interface,
migration fails on the destination guest with assert:

    bdrv_co_pwritev: Assertion `!(bs->open_flags & 0x0800)' failed.

This avoids the problem by delaying the pflash update until after
the device loads complete.

This fix is similar to the one for the pflash_cfi01 migration:

    90c647d Fix pflash migration

Signed-off-by: Laurent Vivier <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: David Gibson <[email protected]>