storage_file: create: Create new images with write permission bit

storage_file: create: Create new images with write permission bit

The 'Create' API of the two storage file backends is used only on
code-paths where we need to format the image after creating an empty
file. Since the DAC security driver only modifies the owner of the file
and not the mode we need to create all files which are going to be
formatted with the write bit set for the user.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>