KVM: remove kvm_read_hva and kvm_read_hva_atomic

The corresponding write functions just use __copy_to_user.  Do the
same on the read side.

This reverts what's left of commit 86ab8cff (KVM: introduce
gfn_to_hva_read/kvm_read_hva/kvm_read_hva_atomic, 2012-08-21)

Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1427976500-28533-1-git-send-email-pbonzini@redhat.com>