KVM: arm64: Sanely ratelimit sysreg messages

Currently kvm_pr_unimpl() is ratelimited, so print_sys_reg_instr() won't
spam the console. However, someof its callers try to print some
contextual information with kvm_err(), which is not ratelimited. This
means that in some cases the context may be printed without the sysreg
encoding, which isn't all that useful.

Let's ensure that both are consistently printed together and
ratelimited, by refactoring print_sys_reg_instr() so that some callers
can provide it with an arbitrary format string.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20191205180652.18671-2-mark.rutland@arm.com