hw/block/nvme: error if drive less than a zone size

hw/block/nvme: error if drive less than a zone size

If a user assigns a backing device with less capacity than the size of a
single zone, the namespace capacity will be reported as zero and the
kernel will silently fail to allocate the namespace.

This patch errors out in case that the backing device cannot accomodate
at least a single zone.

Signed-off-by: Minwoo Im <[email protected]>
[k.jensen: small fixup in the error and commit message]
Signed-off-by: Klaus Jensen <[email protected]>