vmdk: fix metadata write regression

Commit "cdeaf1f vmdk: add bdrv_co_write_zeroes" causes a regression on
writes. It writes metadata after every write instead of doing it only once
for each cluster.

vmdk_pwritev() writes metadata whenever m_data is set as valid so this patch
sets m_data as valid only when we have a new cluster which hasn't been
allocated before or a zero grain.

Signed-off-by: Reda Sallahi <fullmanet@gmail.com>
Message-id: 20160707084249.29084-1-fullmanet@gmail.com
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
