x86 svm: Fix checked builds of Windows running on AMD SVM

x86 svm: Fix checked builds of Windows running on AMD SVM

Checked builds of Windows will, after every modification of the TPR,
read it back again and assert that the value read back matches with
the value written, including the priority sub-class.  Make sure that
we correctly preserve it on vmexit.

As far as I can tell from reading the documentation, the sub-class
doesn't actually do anything, so this should be pretty harmless.

Signed-off-by: Steven Smith <steven.smith@eu.citrix.com>