xen/arm: Save/restore GICH_VMCR on domain context switch

GICH_VMCR register contains alias to important bits of GICV interface such as:
    - priority mask of the CPU
    - EOImode
    - ...

We were safe because Linux guest always use the same value for this bits.
When new guests will handle priority or change EOI mode, VCPU interrupt
management will be in a wrong state.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: George Dunlap <george.dunlap@citrix.com>

xen/arch/arm/gic.c

@@ -107,6 +107,7 @@ void gic_save_state(struct vcpu *v)
         v->arch.gic_lr[i] = GICH[GICH_LR + i];
     v->arch.lr_mask = this_cpu(lr_mask);
     v->arch.gic_apr = GICH[GICH_APR];
+    v->arch.gic_vmcr = GICH[GICH_VMCR];
     /* Disable until next VCPU scheduled */
     GICH[GICH_HCR] = 0;
     isb();
@@ -123,6 +124,7 @@ void gic_restore_state(struct vcpu *v)
     for ( i=0; i<nr_lrs; i++)
         GICH[GICH_LR + i] = v->arch.gic_lr[i];
     GICH[GICH_APR] = v->arch.gic_apr;
+    GICH[GICH_VMCR] = v->arch.gic_vmcr;
     GICH[GICH_HCR] = GICH_HCR_EN;
     isb();