v2v: lib_ovf: Change create_meta_files so it doesn't write the files.

Change Lib_ovf.create_meta_files so that instead of writing the
.meta files, it just returns the content.  Output_rhev and
Output_vdsm are correspondingly changed so that they write the
content to the .meta files.

This is just code refactoring, there is no functional change.
v2v: Allow the output mode to override disk_create.

Also we allow the output mode to actually create the disk image.  This
lets the output mode set ownership and permissions correctly if
required.

The default method just calls Guestfs#disk_create, so there is no
actual change in this commit.
v2v: -o rhev: Write files and directories as user:group 36:36 (RHBZ#1143887).

We need to write files and directories as user:group 36:36, else
RHEV-M cannot import the VM.  Doing this in the presence of NFS is
difficult.  See v2v/kvmuid.mli for how it is done in this commit.
Version 1.27.52.