vmx_set_msr may be called without a ipipe notifier registered, i.e.
outside of vcpu_run. In that case, shared MSRs may be set to guest state
while I-pipe will have no chance to restore them when interrupting
afterwards. Register the notifier and move the whole thing under hard
irq protection.

This fixes crashes when calling "info registers" from the QEMU console a
number times.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>