Add persistent root filesystem

[ivan4th]

If volumeDevices has an entry with 'devicePath: /', it's used as a
persistent root filesystem for the VM. It gets overwritten
again if the image digest changes.
See examples/cirros-vm-persistent-rootfs.yaml

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This change is 

[jellonek]

Reviewed 45 of 45 files at r1.
Reviewable status: 0 of 2 approvals obtained (waiting on @ivan4th)

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pkg/libvirttools/persistentroot_volumesource.go, line 38 at r1 (raw file):

	virtletRootfsMagic           = 0x263dbe52ba576702
	virtletRootfsMetadataVersion = 1
	sectorSize                   = 512

Minor. Will this also work with .iso images as rootfs where block size has 2kb in size?
It's probably not so important, as afair we already lost support for booting from .iso months ago.

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pkg/config/config.go, line 108 at r1 (raw file):

	// this field duplicates glog's --v, so no option for it, which is signified
	// by "+" here (it's only for doc)
	fs.addIntField("logLevel", "+v", "", "Log level to use", logLevelEnv, 6, 0, math.MaxInt32, &c.LogLevel)

It's temporary, right?

[ivan4th]

Reviewable status: 0 of 2 approvals obtained (waiting on @ivan4th)

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pkg/libvirttools/persistentroot_volumesource.go, line 38 at r1 (raw file):

Previously, jellonek (Piotr Skamruk) wrote…

Minor. Will this also work with .iso images as rootfs where block size has 2kb in size?
It's probably not so important, as afair we already lost support for booting from .iso months ago.

For devicemapper it's always 512 according to the docs, and that's what matters here.

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pkg/config/config.go, line 108 at r1 (raw file):

Previously, jellonek (Piotr Skamruk) wrote…

It's temporary, right?

Oops, committed this by accident initially. fixed.

[jellonek]

Reviewed 11 of 11 files at r2.
Reviewable status: 0 of 2 approvals obtained (waiting on @jellonek)

[jellonek]

Reviewable status: 1 of 2 approvals obtained (waiting on @jellonek)

[jellonek]

Reviewed 1 of 1 files at r3.
Reviewable status: 1 of 2 approvals obtained

[pigmej]

Reviewed 34 of 45 files at r1, 9 of 11 files at r2, 1 of 1 files at r3.
Reviewable status: 1 of 2 approvals obtained