arm64: timer: guard ptimer tests with errata

Rather than unconditionally attempting ptimer tests, which
won't work on older KVM, check the KVM version first, reporting
SKIP when the tests would fail. This also allows vtimer and
ptimer tests to be merged into just "timer" tests.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>