Fix sign compare offenders

Fix sign compare offenders

In a few places we compare signed and unsigned integers. In
majority of cases it's because the iteration variable is declared
as ssize_t but then in the for() loop it's compared against an
unsigned int. But there is one case where the opposite happens
(libvirt_virDomainSendKey()), or where an UINT_MAX (which is
inherently unsigned) is compared against signed long.

Also, use this opportunity to decrease scope of 'j' variable
inside of libvirt_virDomainInterfaceAddresses().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>