After updating to kernel 4.1.6_1, I can no longer boot Virtualbox guests if "Use Host I/O Cache" is disabled.
It doesn't appear to matter what the guest is. I have several Void guests for development, and one Windows 7 guest for testing.
All of them are using SATA with AHCI mode as the storage interface.

The specific error Virtualbox throws is:

An error has occurred during virtual machine execution! The error details are shown below. You may try to correct the error and resume the virtual machine execution.
The I/O cache encountered an error while updating data in medium (rc=VERR_INVALID_HANDLE). Make sure there is enough free space on the disk and that the disk is working properly. Operation can be resumed afterwards.

Error ID: BLKCACHE_IOERR
Severity: Non-Fatal Error

The Void host is using the XFS file system. Not sure if that makes a difference (I read about a bug a few years back in the Linux kernel which results in EXT4 users also needing Host I/O Cache enabled, and wonder if it's related).

Anyone else encountered this issue?

Enabling Host I/O Cache is an acceptable workaround (certainly not a deal-breaker), but I would prefer to determine the cause of the issue, and solve it there.
I had disabled it to save on some resources. I'm runnning everything from an SSD, so things are fast enough that I'd rather avoid some of the performance and reliability pitfalls which might occur from using the I/O cahce with multiple VMs running simultaneously.

I'm not certain, just yet, if it's a kernel issue, or if the virtualbox-ose/virtualbox-ose-dkms XBPS package needs an update. I already tried a force-reinstall of the dkms package.
The only thing which corrects the issue is enabling Host I/O Cache.