pvmfw: Support com.android.virt.page_size

Replace the assumption that guests run with 4KiB page tables by adding
support for a new VBMeta descriptor property describing the size in use.
When absent, default to the previous assumption of 4KiB.

Set the property for the Micrdroid kernels with 16KiB PAGE_SIZE.

Add test coverage & document this in the README.

Bug: 339779843
Bug: 339782511
Test: atest libpvmfw_avb.integration_test
Change-Id: Ib3c2b87fd507046578cc95d892d01fa9b04bc5c7
diff --git a/build/microdroid/Android.bp b/build/microdroid/Android.bp
index 7f23ae6..20980e0 100644
--- a/build/microdroid/Android.bp
+++ b/build/microdroid/Android.bp
@@ -560,6 +560,12 @@
             src: ":microdroid_kernel_prebuilt-x86_64",
         },
     },
+    props: [
+        {
+            name: "com.android.virt.page_size",
+            value: "16",
+        },
+    ],
     include_descriptors_from_images: [
         ":microdroid_16k_initrd_normal_hashdesc",
         ":microdroid_16k_initrd_debug_hashdesc",