classes/image_vm: allow different filesystems to be used for VM images.
This allows for things like btrfs to be used vs just ext4.
The default value of ext4 is kept so there is no functional
change unless VM_ROOTFS_TYPE is set in the inherting recipe.
(From OE-Core rev:
df0b217f3df2c36a32e5c4afaec36a28bfc77bbb)
Signed-off-by: Ian Reinhart Geiser <geiseri@geekcentral.pub>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster@mvista.com>
