container's rootfs is a string type, which cannot represent a
block storage backed rootfs which hasn't been mounted.
Change it to a mount alike struct as below:
RootFs struct {
// Source specify the BlockDevice path
Source string
// Target specify where the rootfs is mounted if it has been mounted
Target string
// Type specifies the type of filesystem to mount.
Type string
// Options specifies zero or more fstab style mount options.
Options []string
// Mounted specifies whether the rootfs has be mounted or not
Mounted bool
}
If the container's rootfs has been mounted as before, then this struct can be
initialized as: RootFs{Target: <rootfs>, Mounted: true} to be compatible with
previous case.
Fixes:#1158
Signed-off-by: lifupan <lifupan@gmail.com>
