Apart from the legacy case, you need it for MMIO---KVM for ARM also has a mini parser for LDR/STR instructions.
x86 however has all sorts of wonderful read-modify-write instructions too. You need to support those, but it would still be a small subset of the full x86 instruction set if you all you want to support is processors newer than circa 2010.
KVM for ARM doesn't parse instructions -- you can just use the register info the hardware gives you in the syndrome register, which covers everything except oddball cases like trying load-multiple to a device, which doesn't happen in practice and so we don't support it.
x86 however has all sorts of wonderful read-modify-write instructions too. You need to support those, but it would still be a small subset of the full x86 instruction set if you all you want to support is processors newer than circa 2010.