qemu-nbd: Avoid strtol open-coding

qemu-nbd: Avoid strtol open-coding

Our copy-and-pasted open-coding of strtol handling forgot to
handle overflow conditions.  Use qemu_strto*() instead.

In the case of --partition, since we insist on a user-supplied
partition to be non-zero, we can use 0 rather than -1 for our
initial value to distinguish when a partition is not being
served, for slightly more optimal code.

The error messages for out-of-bounds values are less specific,
but should not be a terrible loss in quality.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Richard W.M. Jones <[email protected]>
Message-Id: <20190117193658[email protected]>