kvmtool: 9p: fix check for snprintf truncation of full_path

The check on the return value of snprintf should reuse the size parameter,
rather than take sizeof(full_path) as the bound.

Signed-off-by: G. Campana <gcampana+kvm@quarkslab.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>