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 <minwoo.im.dev@gmail.com>
[k.jensen: small fixup in the error and commit message]
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>