8 years later I'm here to answer this question.
Default FreeBSD ISO/etc does not enable serial port by default(As far as I know).
I solved it by building a custom ISO.
1) Install FreeBSD using virt-...
This is not KVM/QEMU/LIBVIRT bug, since it is the kernel running out of system memory by allocating of 1G.
This can be:
impossibility to free cached blocks by the FS (depending on type of FS, e. g. ...