KVM: arm64: nvhe: Disable branch generation in nVHE guests

This adds a new vcpu flag i.e DEBUG_STATE_SAVE_BRBE to detect running BRBE
instance in the host, save its controls when transitioning into the guest,
and restore back those control when transitioning back into the host.

Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: James Morse <james.morse@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: kvmarm@lists.linux.dev
Cc: linux-arm-kernel@lists.infradead.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>