KVM: get rid of $(addprefix ../../../virt/kvm/, ...) in Makefiles

As requested by the KVM maintainers, remove the addprefix used to
refer to the main KVM code from the arch code, and replace it with
a KVM variable that does the same thing.

Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: Christoffer Dall <cdall@cs.columbia.edu>
Acked-by: Xiantao Zhang <xiantao.zhang@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>