lib: guestfs_disk_create: Allow vmdk as a valid backingformat.

Commit 588af1953e5f7ab74009b9175cc5d3efb8bb651a started with a very
conservative list of permitted backing formats (just "raw" or
"qcow2").  We can allow almost any format permitted by qemu, but this
commit just adds "vmdk" to this whitelist.
ntfsresize: Capture errors sent to stdout (RHBZ#1166618).

ntfsresize sends error messages to stdout.  Capture those error
messages.
v2v: -i ova: Remove incorrect warning for disks that have no parent controller (RHBZ#1167302).

Don't assume every disk <Item> has a <Parent> field.  For floppy disks
this is not the case.

Thanks: Junqin Zhou