x86/vmx: Don't use hvm_inject_hw_exception() in long_mode_do_msr_write()

x86/vmx: Don't use hvm_inject_hw_exception() in long_mode_do_msr_write()

Since c/s 49de10f3c1718 "x86/hvm: Don't raise #GP behind the emulators back
for MSR accesses", returning X86EMUL_EXCEPTION has pushed the exception
generation to the top of the call tree.

Using hvm_inject_hw_exception() and returning X86EMUL_EXCEPTION causes a
double #GP injection, which combines to #DF.

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