qemu-ga: avoid blocking on atime update when reading /etc/mtab

Currently we re-read/re-process /etc/mtab to get an updated list of
mounts when guest-fsfreeze-thaw is called. This can cause an atime
update on /etc/mtab, which will block if we're in a frozen state.

Instead, use /proc's version of mtab, which may not be up-to-date with
options passed via -o remount, but is compatible for our use cases since
we only care about the filesystem type.
Reported-by: NMatsuda, Daiki <matsudadik@intellilink.co.jp>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qga/commands-posix.c

@@ -345,7 +345,7 @@ static int guest_fsfreeze_build_mount_list(GuestFsfreezeMountList *mounts)
 {
     struct mntent *ment;
     GuestFsfreezeMount *mount;
-    char const *mtab = MOUNTED;
+    char const *mtab = "/proc/self/mounts";
     FILE *fp;
 
     fp = setmntent(mtab, "r");