tests: Add 'kvm' tag to tests

Some of our tests require KVM. Tagging these tests as such allow us to
filter them in or out during testing.

Change-Id: I3f188d3e268409535043cc32b64d99e4657638cd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56829
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

ext/testlib/configuration.py

@@ -254,6 +254,8 @@ def define_constants(constants):
     constants.host_x86_64_tag = 'x86_64'
     constants.host_arm_tag = 'aarch64'
 
+    constants.kvm_tag = 'kvm'
+
     constants.supported_tags = {
         constants.isa_tag_type : (
             constants.x86_tag,