plugin: return number of supported MSRS even if buffer is too small
plugin: return number of supported MSRS even if buffer is too small

Userspace is interested in number of supported MSRs even if supplied
buffer is too small, as then it can intelligently [re]allocate the
buffer and repeat the call instead of doing this blindly. So let's
always populate 'out_count'in crosvm_get_msr_index_list() call.
Obviously if there is hard error we will not be able to supply a
meaningful number, so 0 will be returned, but in case of E2BIG error we
can return the real number.

Also let's do the same for get_supported_cpuids() and
get_emulated_cpuids() calls.

BUG=b:111083877
TEST=cargo test -p kvm; cargo test --features=plugin

Change-Id: I37a8d719103fac44597b88ddecb6b8af2dd54ac8
Signed-off-by: Dmitry Torokhov <10a8c465cefc9bdd6c925e26964d23c90f1141cc@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1185293
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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