When migrating ARM guests accross same machines with different host
kernels we are likely to encounter failures such as:

"failed to load cpu:cpreg_vmstate_array_len"

This is due to the fact KVM exposes a different number of registers
to qemu on source and destination. When trying to migrate a bigger
register set to a smaller one, qemu cannot save the CPU state.

For example, recently we faced such kind of situations with:

• unconditionnal exposure of KVM_REG_ARM_VENDOR_HYP_BMAP_2 FW pseudo
  register from v6.16 onwards. Causes backward migration failure.
• removal of unconditionnal exposure of TCR2_EL1, PIRE0_EL1, PIR_EL1
  from v6.13 onwards. Causes forward migration failure.