eal: fix hotplug device parsing

eal: fix hotplug device parsing

snprintf returns the length it would have written had the given length
been enough, *terminating null byte excluded*.

It will however limit the length of its writing to given length minus
one, and always put a terminating null-byte at the end of the string.

This must be taken into account when calculating the total length of the
device declaration string.

Fixes: 3054036f054a ("eal: fix possible crash in hotplug")

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>