vmtemplate: allow allocation = 0 for type 'raw'

Kimchi was allocating the entire disk when the disk image type wasn't
'qcow2'. Adding 'raw' to this exception list because raw images can
be created as sparse (= size in bytes when creating the disk is
less than its total capacity) too.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>