x86/traps: make panic and reboot paths safe during early boot

x86/traps: make panic and reboot paths safe during early boot

Reverse two conditions in show_registers().  For an early crash, it is not
safe to dereference 'current' for its HVM status before knowing that it is a
guest vcpu.

Introduce SYS_STATE_smp_boot to distinguish the point at which APs need
considering before boot is complete.  There is one code change required as a
result; .init.text symbols are still in use before Xen is active, so alter its
predicate in is_active_kernel_text().

Make use of SYS_STATE_smp_boot in machine_{halt,restart}().  Before Xen starts
booting the APs, any execution here is certainly the BSP.

When halting or rebooting particularly early, this avoids the risks of a #PF
or #GP when accessing the LAPIC before generic_apic_probe(), as well as trying
to enable interrupts before init_IRQ() is complete.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>