x86: Implement AMD-V NMI handler and CPU reset

x86: Implement AMD-V NMI handler and CPU reset

NMIs originate from the hypervisor which uses them to manage cells. As a part
of this management task, the CPU can be reset. This is performed in vcpu_reset().

Jailhouse runs with GIF set, and it needs to be cleared shortly to let the CPU
to consume pending interrupt and allow further NMIs trigger VM exits. Otherwise,
current NMI would be delivered to the guest on the next VM entry.

NMI can trigger in host mode only if Jailhouse explicitly clears the GIF to
consume the pending interrupt. There is nothing to handle in this case, so
vcpu_nmi_handler() simply prints a message that the NMI was consumed.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>