vmdk: Check VMFS extent line field number

VMFS extent line in description file should be with 4 fields:

    RW <size> VMFS "file-name.vmdk"

Check the number explicitly and report error if offset is appended as
FLAT, which should be invalid format.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
