qemu-io: add cvtnum() error handling for zone commands

cvtnum() parses positive int64_t values and returns a negative errno on
failure. Print errors and return early when cvtnum() fails.

While we're at it, also reject nr_zones values greater or equal to 2^32
since they cannot be represented.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Sam Li <faithilikerun@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240507180558.377233-1-stefanha@redhat.com>
Reviewed-by: Sam Li <faithilikerun@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>