qemu-ga: qmp_guest_network_get_interfaces(): get rid of snprintf() + error_set()

Convert them to error_setg_errno().

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
