Update following V1.BLOCK error type change

We now return Error Is_read_only for an attempt to write
to a read/only device, and Error Unimplemented otherwise.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Flesh out V1.BLOCK read, write

Note we advertise a minimum sector size of 4KiB (ie a page) to ensure
the read/write start_sector is page-aligned as well as sector-aligned.
More and more disks are using 4kiB sectors natively, and this simplifies
the logic.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>