arch-arm: Fix gem5 build on aarch64 host

vm is not a member variable of MuxingKvmGic. This was broken
by:

https://gem5-review.googlesource.com/c/public/gem5/+/56263

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Change-Id: Iee8a3424eb28cfe2bc20df088dc0af05e9e8a7de
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56346
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/kvm/gic.cc

@@ -190,6 +190,8 @@ void
 MuxingKvmGic::startup()
 {
     GicV2::startup();
+
+    KvmVM *vm = system.getKvmVM();
     usingKvm = kernelGic && vm && vm->validEnvironment();
     if (usingKvm)
         fromGicToKvm();
@@ -207,6 +209,8 @@ void
 MuxingKvmGic::drainResume()
 {
     GicV2::drainResume();
+
+    KvmVM *vm = system.getKvmVM();
     bool use_kvm = kernelGic && vm && vm->validEnvironment();
     if (use_kvm != usingKvm) {
         // Should only occur due to CPU switches