src: fix redundant/misleading build conditional for KVM MSR code

src: fix redundant/misleading build conditional for KVM MSR code

The condition

   WITH_LINUX_KVM_H && (defined(__linux__) || defined(__FreeBSD__))

is redundant. If the meson check for linux/kvm.h succeeded, we
must be on a Linux host and cannot be on a FreeBSD host. Remove
these redundant OS conditions from the MSR code to stop misleading
readers.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>