remove KVM_CAP_MAX_VCPUS hack

As we now have the header file in our repository, we can safely follow
the recommendation in kvm.c and remove the hack adding the
KVM_CAP_MAX_VCPUS macro.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>