qemu: block: Validate node-names for use with qemu

qemu: block: Validate node-names for use with qemu

qemu declares node-name as a 32 byte buffer and silently truncates
anything longer than that. This is unacceptable for libvirt, so we need
to make sure that we won't ever supply a node-name exceeding 31 chars.

Add a function which will do the validation and use it to validate
storage-protocol node names.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>