[zx] Make all zx::guest and zx::vcpu methods const

Since we only care about C++ const-correctness, and not whether the
operation is mutating the kernel object, we should mark these methods as
const.

Test: Ran system/utest/hypervisor.
Change-Id: Ic10e9b86a24dd7a91a49d88efc2c4f98a78ce756
diff --git a/system/ulib/zx/include/lib/zx/guest.h b/system/ulib/zx/include/lib/zx/guest.h
index 037bc6d..f7ab91b 100644
--- a/system/ulib/zx/include/lib/zx/guest.h
+++ b/system/ulib/zx/include/lib/zx/guest.h
@@ -34,7 +34,7 @@
                               guest* guest, vmar* vmar);
 
     zx_status_t set_trap(uint32_t kind, zx_gpaddr_t addr, size_t len,
-                         const port& port, uint64_t key) {
+                         const port& port, uint64_t key) const {
         return zx_guest_set_trap(get(), kind, addr, len, port.get(), key);
     }
 };
diff --git a/system/ulib/zx/include/lib/zx/vcpu.h b/system/ulib/zx/include/lib/zx/vcpu.h
index dac3f3a..a38b754 100644
--- a/system/ulib/zx/include/lib/zx/vcpu.h
+++ b/system/ulib/zx/include/lib/zx/vcpu.h
@@ -32,11 +32,11 @@
     static zx_status_t create(const guest& guest, uint32_t options,
                               zx_gpaddr_t entry, vcpu* result);
 
-    zx_status_t resume(zx_port_packet_t* packet) {
+    zx_status_t resume(zx_port_packet_t* packet) const {
         return zx_vcpu_resume(get(), packet);
     }
 
-    zx_status_t interrupt(uint32_t interrupt) {
+    zx_status_t interrupt(uint32_t interrupt) const {
         return zx_vcpu_interrupt(get(), interrupt);
     }
 
@@ -44,7 +44,7 @@
         return zx_vcpu_read_state(get(), kind, buf, len);
     }
 
-    zx_status_t write_state(uint32_t kind, const void* buf, size_t len) {
+    zx_status_t write_state(uint32_t kind, const void* buf, size_t len) const {
         return zx_vcpu_write_state(get(), kind, buf, len);
     }
 };