virtio-serial: avoid crash when port has no name

It seems "name" is not mandatory, and the following command line (based
on one generated by current libvirt) will crash qemu at start:

qemu-system-x86_64 \
    -device virtio-serial-pci \
    -device virtserialport,name=foo \
    -device virtconsole

Program received signal SIGSEGV, Segmentation fault.
__strcmp_ssse3 () at ../sysdeps/x86_64/strcmp.S:210
210        movlpd    (%rsi), %xmm2
Missing separate debuginfos, use: debuginfo-install
python-libs-2.7.5-13.fc20.x86_64
(gdb) bt
 #0  __strcmp_ssse3 () at ../sysdeps/x86_64/strcmp.S:210
 #1  0x000055555566bdc6 in find_port_by_name (name=0x0) at /home/elmarco/src/qemu/hw/char/virtio-serial-bus.c:67

Signed-off-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
