x86/svm: handle BU_CFG and BU_CFG2 with cases

x86/svm: handle BU_CFG and BU_CFG2 with cases

Move the special handling of reads to it's own switch case, and also
add support for BU_CFG2. On the write side ignore writes if the MSR is
readable, otherwise return a #GP.

This is in preparation for changing the default MSR read/write
behavior, which will instead return #GP on not explicitly handled
cases.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>