x86/HVM: don't give the wrong impression of WRMSR succeeding

x86/HVM: don't give the wrong impression of WRMSR succeeding

... for non-existent MSRs: wrmsr_hypervisor_regs()'s comment clearly
says that the function returns 0 for unrecognized MSRs, so
{svm,vmx}_msr_write_intercept() should not convert this into success. We
don't want to unconditionally fail the access though, as we can't be
certain the list of handled MSRs is complete enough for the guest types
we care about, so instead mirror what we do on the read paths and probe
the MSR to decide whether to raise #GP.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>