x86: fix reboot/shutdown with running HVM guests

x86: fix reboot/shutdown with running HVM guests

If there's a guest using VMX/SVM when the hypervisor shuts down, it
can lead to the following crash due to VMX/SVM functions being called
after hvm_cpu_down has been called. In order to prevent that, check in
{svm/vmx}_ctxt_switch_from that the cpu virtualization extensions are
still enabled.

(XEN) Domain 0 shutdown: rebooting machine.
(XEN) Assertion 'read_cr0() & X86_CR0_TS' failed at vmx.c:644
(XEN) ----[ Xen-4.5-unstable  x86_64  debug=y  Tainted:    C ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff82d0801d90ce>] vmx_ctxt_switch_from+0x1e/0x14c
...
(XEN) Xen call trace:
(XEN)    [<ffff82d0801d90ce>] vmx_ctxt_switch_from+0x1e/0x14c
(XEN)    [<ffff82d08015d129>] __context_switch+0x127/0x462
(XEN)    [<ffff82d080160acf>] __sync_local_execstate+0x6a/0x8b
(XEN)    [<ffff82d080160af9>] sync_local_execstate+0x9/0xb
(XEN)    [<ffff82d080161728>] map_domain_page+0x88/0x4de
(XEN)    [<ffff82d08014e721>] map_vtd_domain_page+0xd/0xf
(XEN)    [<ffff82d08014cda2>] io_apic_read_remap_rte+0x158/0x29f
(XEN)    [<ffff82d0801448a8>] iommu_read_apic_from_ire+0x27/0x29
(XEN)    [<ffff82d080165625>] io_apic_read+0x17/0x65
(XEN)    [<ffff82d080166143>] __ioapic_read_entry+0x38/0x61
(XEN)    [<ffff82d080166aa8>] clear_IO_APIC_pin+0x1a/0xf3
(XEN)    [<ffff82d080166bae>] clear_IO_APIC+0x2d/0x60
(XEN)    [<ffff82d080166f63>] disable_IO_APIC+0xd/0x81
(XEN)    [<ffff82d08018228b>] smp_send_stop+0x58/0x68
(XEN)    [<ffff82d080181aa7>] machine_restart+0x80/0x20a
(XEN)    [<ffff82d080181c3c>] __machine_restart+0xb/0xf
(XEN)    [<ffff82d080128fb9>] smp_call_function_interrupt+0x99/0xc0
(XEN)    [<ffff82d080182330>] call_function_interrupt+0x33/0x43
(XEN)    [<ffff82d08016bd89>] do_IRQ+0x9e/0x63a
(XEN)    [<ffff82d08016406f>] common_interrupt+0x5f/0x70
(XEN)    [<ffff82d0801a8600>] mwait_idle+0x29c/0x2f7
(XEN)    [<ffff82d08015cf67>] idle_loop+0x58/0x76
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Assertion 'read_cr0() & X86_CR0_TS' failed at vmx.c:644
(XEN) ****************************************

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>