grow disk: call SetDiskID before blockdev_getsize
grow disk: call SetDiskID before blockdev_getsize

Missing this call caused blockdev_getsize to be called on the wrong
physical device, and fail.

This fixes Issue 497.

Signed-off-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

onponomarev/ganeti,andir/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti,andir/ganeti,dimara/ganeti,yiannist/ganeti,grnet/snf-ganeti,yiannist/ganeti,leshchevds/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,andir/ganeti,apyrgio/snf-ganeti,dimara/ganeti,bitemyapp/ganeti,ganeti/ganeti,leshchevds/ganeti,onponomarev/ganeti,mbakke/ganeti,leshchevds/ganeti