KVM: SVM: adjust register allocation for __svm_vcpu_run()

32-bit ABI uses RAX/RCX/RDX as its argument registers, so they are in
the way of instructions that hardcode their operands such as RDMSR/WRMSR
or VMLOAD/VMRUN/VMSAVE.

In preparation for moving vmload/vmsave to __svm_vcpu_run(), keep
the pointer to the struct vcpu_svm in %rdi.  In particular, it is now
possible to load svm->vmcb01.pa in %rax without clobbering the struct
vcpu_svm pointer.

No functional change intended.

Cc: stable@vger.kernel.org
Fixes: a149180f ("x86: Add magic AMD return-thunk")
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>