vmdk: fix cluster size check for flat extents

We use the extent size as cluster size for flat extents (where no L1/L2
table is allocated so it's safe) reuse sector calculating code with
sparse extents.

Don't pass in the cluster size for adding flat extent, just set it to
sectors later, then the cluster size checking will not fail.

The cluster_sectors is changed to int64_t to allow big flat extent.

Without this, flat extent opening is broken:

    # qemu-img create -f vmdk -o subformat=monolithicFlat /tmp/a.vmdk 100G
    Formatting '/tmp/a.vmdk', fmt=vmdk size=107374182400 compat6=off subformat='monolithicFlat' zeroed_grain=off
    # qemu-img info /tmp/a.vmdk
    image: /tmp/a.vmdk
    file format: raw
    virtual size: 0 (0 bytes)
    disk size: 4.0K

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>