x86/svm: Don't clobber eax and edx if an RDMSR intercept fails

x86/svm: Don't clobber eax and edx if an RDMSR intercept fails

The original code has a bug; eax and edx get unconditionally updated even when
hvm_msr_read_intercept() doesn't return X86EMUL_OKAY.

It is only by blind luck (vmce_rdmsr() eagerly initialising its msr_content
pointer) that this isn't an information leak into guests.

While fixing this bug, reduce the scope of msr_content and initialise it to 0.
This makes it obvious that a stack leak won't occur, even if there were to be
a buggy codepath in hvm_msr_read_intercept().

Also make some non-functional improvements.  Make the insn_len calculation
common, and reduce the quantity of explicit casting by making better use of
the existing register names.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>