smp: speed up cpu_count()

cpu_count() is used in important places, like vmexit.flat's measuring
loop, yet it is ridiculously slow as it talks to the firmware config
interface.

Speed it up by reading the value from memory.
Signed-off-by: Avi Kivity <avi@redhat.com>

lib/x86/smp.c

@@ -78,7 +78,7 @@ void spin_unlock(struct spinlock *lock)
 
 int cpu_count(void)
 {
-    return fwcfg_get_nb_cpus();
+    return _cpu_count;
 }
 
 int smp_id(void)
@@ -130,6 +130,8 @@ void smp_init(void)
     int i;
     void ipi_entry(void);
 
+    _cpu_count = fwcfg_get_nb_cpus();
+
     set_ipi_descriptor(ipi_entry);
 
     setup_smp_id(0);