arm64: KVM: Reserve 4 additional instructions in the BPI template

So far, we only reserve a single instruction in the BPI template in
order to branch to the vectors. As we're going to stuff a few more
instructions there, let's reserve a total of 5 instructions, which
we're going to patch later on as required.

We also introduce a small refactor of the vectors themselves, so that
we stop carrying the target branch around.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>