virDomainVirtioSerialAddrSetFree: Fix memleak
    
    ==19015== 8 bytes in 1 blocks are definitely lost in loss record 34 of 1,049
    ==19015==    at 0x4C29F80: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==19015==    by 0x4C2C32F: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==19015==    by 0x52AD888: virReallocN (viralloc.c:245)
    ==19015==    by 0x52AD97E: virExpandN (viralloc.c:294)
    ==19015==    by 0x52ADC51: virInsertElementsN (viralloc.c:436)
    ==19015==    by 0x5335864: virDomainVirtioSerialAddrSetAddController (domain_addr.c:816)
    ==19015==    by 0x53358E0: virDomainVirtioSerialAddrSetAddControllers (domain_addr.c:839)
    ==19015==    by 0x1DD5513B: qemuDomainAssignVirtioSerialAddresses (qemu_command.c:1422)
    ==19015==    by 0x1DD55A6E: qemuDomainAssignAddresses (qemu_command.c:1711)
    ==19015==    by 0x1DDA5818: qemuProcessStart (qemu_process.c:4616)
    ==19015==    by 0x1DDF1807: qemuDomainObjStart (qemu_driver.c:7265)
    ==19015==    by 0x1DDF1A66: qemuDomainCreateWithFlags (qemu_driver.c:7320)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>