vmdk: Fix data corruption bug in WRITE and READ handling

Fixed a MAJOR BUG in VMDK files on file boundaries on reads
and ALSO ON WRITES WHICH MIGHT CORRUPT THE IMAGE AND DATA!!!!!!

Triggered for example with the following VMDK file (partly listed):
RW 4193792 FLAT "XP-W1-f001.vmdk" 0
RW 2097664 FLAT "XP-W1-f002.vmdk" 0
RW 4193792 FLAT "XP-W1-f003.vmdk" 0
RW 512 FLAT "XP-W1-f004.vmdk" 0
RW 4193792 FLAT "XP-W1-f005.vmdk" 0
RW 2097664 FLAT "XP-W1-f006.vmdk" 0
RW 4193792 FLAT "XP-W1-f007.vmdk" 0
RW 512 FLAT "XP-W1-f008.vmdk" 0

Patch includes:
1.) Patch fixes wrong calculation on extent boundaries. Especially it
fixes the relativeness of the sector number to the current extent.

Verfied correctness with:
1.) Converted either with Virtualbox to VDI and then with qemu-img and
    then with qemu-img only:

    VBoxManage clonehd --format vdi /VM/XP-W/new/XP-W1.vmdk ~/.VirtualBox/Harddisks/XP-W1-new-test.vdi
    ./qemu-img convert -O raw ~/.VirtualBox/Harddisks/XP-W1-new-test.vdi /root/QEMU/VM-XP-W1/XP-W1-via-VBOX.img
    md5sum /root/QEMU/VM-XP-W/XP-W1-direct.img
    md5sum /root/QEMU/VM-XP-W/XP-W1-via-VBOX.img
    => same MD5 hash

2.) Verified debug log files
3.) Run Windows XP successfully
4.) chkdsk run successfully without any errors

Signed-off-by: Gerhard Wiesinger <lists@wiesinger.com>
Acked-by: Fam Zheng <famcool@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
