guestfs: create mount folder if initially computed one is busy

[cbosdo]

What does this PR do?

When mounting an image, if the computed folder is already existing and
not empty, then we try to find another one that could be used. But then
we need to create the folder if needed.

What issues does this PR fix or reference?

Issue #55349.

Previous Behavior

Running this fails:

mkdir -p /tmp/guest/var.testsuite-data.disk-image-template.qcow2/foobar
salt-call --local guestfs.mount /var/testsuite-data/disk-image-template.qcow2

New Behavior

Running the same mounts the image without error

Tests written?

Yes

Commits signed with GPG?

Yes

[cbosdo]

@waynew since you were involved in the issue, you may want to review that PR

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ad100e2). Click here to learn what that means.
The diff coverage is 60%.

@@            Coverage Diff            @@
##             master   #55672   +/-   ##
=========================================
  Coverage          ?   39.15%           
=========================================
  Files             ?     1466           
  Lines             ?   260246           
  Branches          ?    56399           
=========================================
  Hits              ?   101866           
  Misses            ?   147200           
  Partials          ?    11180

Flag	Coverage Δ
#amazon2	37.76% <40%> (?)
#debian8	38.25% <40%> (?)
#debian9	38.23% <40%> (?)
#fedora31	38.28% <40%> (?)
#py2	38.28% <40%> (?)
#py3	38.34% <40%> (?)
#runtests	39.15% <60%> (?)
#zeromq	39.15% <60%> (?)

Impacted Files	Coverage Δ
salt/modules/guestfs.py	80.56% <60%> (ø)

[dwoz]

re-run full centos