backup: default tmpdir to /tmp, respect TMPDIR

This requires having at least 1GB free on /tmp, but it is fair
assumption - it's tmpfs in dom0 and while performing the backup most of
the VMs aren't running, so shouldn't be a problem. Anyway it is always
possible to set TMPDIR variable or pass --tmpdir cmdline option.
Using tmpfs based temporary directory should speedup the backup.

QubesOS/qubes-issues#1652

core/backup.py

@@ -450,7 +450,7 @@ def backup_do(base_backup_dir, files_to_backup, passphrase,
               progress_callback=None, encrypted=False, appvm=None,
               compressed=False, hmac_algorithm=DEFAULT_HMAC_ALGORITHM,
               crypto_algorithm=DEFAULT_CRYPTO_ALGORITHM,
-              tmpdir="/var/tmp"):
+              tmpdir=None):
     global running_backup_operation
 
     def queue_put_with_check(proc, vmproc, queue, element):
@@ -511,7 +511,7 @@ def backup_do(base_backup_dir, files_to_backup, passphrase,
         progress = blocks_backedup * 11 / total_backup_sz
         progress_callback(progress)
 
-    backup_tmpdir = tempfile.mkdtemp(prefix="{}/backup_".format(tmpdir))
+    backup_tmpdir = tempfile.mkdtemp(prefix="backup_", dir=tmpdir)
     running_backup_operation.tmpdir_to_remove = backup_tmpdir
 
     # Tar with tape length does not deals well with stdout (close stdout between