arm64: add vtimer test

All virtualization capable ARM cores support the ARM virtual timer.

Add minimalistic check for firing a virtual timer event, confirming
by checking that the timer pin is marked as pending on the GIC.

Signed-off-by: Alexander Graf <agraf@suse.de>
[ Applied my review comments and a few other tweaks. ]
Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20170704135838.9061-2-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>