arm/run: ensure we use processor=host for kvm

The configure script will happily default to cortex-a15 even when
running on a cortex-a7. This runs into problems when running KVM on
common dev boards as we should be using the "host" processor.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20170518160208.9674-3-alex.bennee@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>