Create virtmgr domain and initial policy

Start a new security domain for virtmgr - a child proces of an app that
manages its virtual machines.

Add permissions to auto-transition to the virtmgr domain when the client
fork/execs virtmgr and to communicate over UDS and pipe.

Bug: 250685929
Test: atest -p packages/modules/Virtualization:avf-presubmit
Change-Id: I7624700b263f49264812e9bca6b83a003cc929be
diff --git a/apex/com.android.virt-file_contexts b/apex/com.android.virt-file_contexts
index cc712ff..caef62d 100644
--- a/apex/com.android.virt-file_contexts
+++ b/apex/com.android.virt-file_contexts
@@ -1,4 +1,5 @@
 (/.*)?                     u:object_r:system_file:s0
 /bin/crosvm                u:object_r:crosvm_exec:s0
 /bin/fd_server             u:object_r:fd_server_exec:s0
+/bin/virtmgr               u:object_r:virtmgr_exec:s0
 /bin/virtualizationservice u:object_r:virtualizationservice_exec:s0