xl: NULL terminate buf when reading dom0 /proc/uptime

xl: NULL terminate buf when reading dom0 /proc/uptime

The contents of /proc/uptime is typically something like "80164.57
640617.58", so the existing 512 byte buffer is more than large enoguh,
so reduce its effective size to 511 bytes and ensure we include a
NULL.

Otherwise Coverity points out that we pass a potentially unterminated
string to strtok. In practice this likely doesn't actually cause
issues (at least on Linux) because the
string should always contain a space so we will stop parsing.

CID: 105590

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>