KVM: selftests: get-reg-list: Add KVM_REG_ARM_FW_REG(3)

KVM: selftests: get-reg-list: Add KVM_REG_ARM_FW_REG(3)

When testing a kernel with commit fb69b29fcaf0 ("KVM: arm64:
Allow SMCCC_ARCH_WORKAROUND_3 to be discovered and migrated")
get-reg-list output

vregs: Number blessed registers:   234
vregs: Number registers:           238

vregs: There are 1 new registers.
Consider adding them to the blessed reg list with the following lines:

	KVM_REG_ARM_FW_REG(3),

vregs: PASS
...

That output inspired two changes: 1) add the new register to the
blessed list and 2) explain why "Number registers" is actually four
larger than "Number blessed registers" (on the system used for
testing), even though only one register is being stated as new.
The reason is that some registers are host dependent and they get
filtered out when comparing with the blessed list. The system
used for the test apparently had three filtered registers.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220316125129.392128-1-drjones@redhat.com