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fix(kvm): check the result and chunk size in kvm_set_msrs

KvmVcpu::kvm_set_msrs() had two bugs:

- KVM_SET_MSRS stops at the first MSR that it cannot write and returns
  the number of MSRs written, instead of an error. kvm_set_msrs()
  discarded that count, so an unsupported or invalid MSR silently
  skipped itself and the rest of the slice. Check the count and fail
  with Error::SetMsr { index }.
- KVM accepts at most MAX_IO_MSRS - 1 entries per call: a call with
  MAX_IO_MSRS entries failed with E2BIG, and a longer slice panicked on
  an out-of-bounds index. Split the slice into chunks of MSRS_PER_CALL,
  which the KVM bindings now export instead of MAX_IO_MSRS.

Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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