vma: allow partial restore

vma: allow partial restore

Introduce a new map line for skipping a certain drive, of the form
skip=drive-scsi0

Since in PVE, most archives are compressed and piped to vma for
restore, it's not easily possible to skip reads.

For the reader, a new skip flag for VmaRestoreState is added and the
target is allowed to be NULL if skip is specified when registering.
If
the skip flag is set, no writes will be made as well as no check for
duplicate clusters. Therefore, the flag is not set for verify.

Originally-by: Fabian Ebner <f.ebner@proxmox.com>
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
Acked-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>