kvm: annotate tests for x86-specific APIs
kvm: annotate tests for x86-specific APIs

Not all tests can run on arches other than x86, annotate them
accordingly.

BUG=None
TEST=cargo test -p kvm

Change-Id: I38c90f1431b7fbd9059f5c309dc28264bbbcdb99
Signed-off-by: Dmitry Torokhov <10a8c465cefc9bdd6c925e26964d23c90f1141cc@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1419206
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm