better provide a interface for linuxBlockIODevice

[enzoyes]

hi, when i try to modify linuxBlockIODevice outside package, it seems not convenient for its anonymous.
code slice is below.
What about provide a interface like GetlinuxBlockIODevice here or make it public?
If ok, i will provide a merge later.

// linuxBlockIODevice holds major:minor format supported in blkio cgroup
type linuxBlockIODevice struct {
// Major is the device's major number.
Major int64 json:"major"
// Minor is the device's minor number.
Minor int64 json:"minor"
}
// LinuxThrottleDevice struct holds a major:minor rate_per_second pair
type LinuxThrottleDevice struct {
linuxBlockIODevice
// Rate is the IO rate limit per cgroup per device
Rate uint64 json:"rate"
}

THX.