KVM: selftests: Convert userspace_msr_exit_test away from VCPU_ID

Convert userspace_msr_exit_test to use vm_create_with_one_vcpu() and pass
around a 'struct kvm_vcpu' object instead of using a global VCPU_ID.
Note, this is a "functional" change in the sense that the test now
creates a vCPU with vcpu_id==0 instead of vcpu_id==1.  The non-zero
VCPU_ID was 100% arbitrary and added little to no validation coverage.
If testing non-zero vCPU IDs is desirable for generic tests, that can be
done in the future by tweaking the VM creation helpers.

Opportunistically use vcpu_run() instead of _vcpu_run() with an open
coded assert that KVM_RUN succeeded.  Fix minor coding style violations
too.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>