KVM: x86 emulator: tighen up ->read_std() and ->write_std() error checks

Instead of checking for X86EMUL_PROPAGATE_FAULT, check for any error,
making the callers more reliable.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>