fix storage base size support

[bergwolf]

When set, it is used to set both rootfs and volume size for rawblock and devicemapper graph drivers.

e.g., having StorageBaseSize=3gb:

[root@lear hyper]# hyperctl run -v /data --rm -t busybox
/ # df -h
Filesystem                Size      Used Available Use% Mounted on
/dev/sdb                  3.0G     33.4M      3.0G   1% /
devtmpfs                 53.5M         0     53.5M   0% /dev
tmpfs                    58.1M         0     58.1M   0% /dev/shm
/dev/sda                  3.0G     32.2M      3.0G   1% /data
share_dir                 1.0M      4.0K   1020.0K   0% /etc/hosts

cc @minz1027

[laijs]

LGTM

[minzcmu]

Thanks for fixing it! This feature is really helpful!