selftests: kvm: list once tests that are valid on all architectures

Several tests cover infrastructure from virt/kvm/ and userspace APIs that have
only minimal requirements from architecture-specific code.  As such, they are
available on all architectures that have libkvm support, and this presumably
will apply also in the future (for example if loongarch gets selftests support).
Put them in a separate variable and list them only once.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250401141327.785520-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>