revert merge 2ef774c98e12aa072d3cd0d0ec90971521aaef93

This commit reverts merge 2ef774c98e12aa072d3cd0d0ec90971521aaef93,
which contains fixes for regressions introduced by merge
0f0a4809d8df4ec31ff1d2096b2a2e5fc9784c4c (CP-6697). We are revert this
merge because we don't have time to test it. The commits reverted are:
* 6cf9efdef7984aec28e51f5a45f2ad3373fa3a8b: "CA-125122: Ensure the existance
  of remotepath when calling _isbind"
* f03332c3c82f1b9d893cf3328d6a166febd6e502: "CA-125137: sr_ref doesn't always
  exist"

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
revert merge 0f0a4809d8df4ec31ff1d2096b2a2e5fc9784c4c

This commit reverts merge 0f0a4809d8df4ec31ff1d2096b2a2e5fc9784c4c
(CP-6697) because that merge introduced significant regressions. The
commits that are reverted are:
 * eed0ec8f4b2fc01e97e49a1bd5913501b7b53c39: "CP-6697: normal ismount can not
   deal with bind mount"
 * 34b9bb382e06ad952fb491446b47d65e6f64860d: "CP-6697: Modernize FileSR as an
   independent SR type"
 * 691cd9782b28eccd5d6662b34ea8fae42c8cb6cb: "CP-6697: Refactor _checkmount to
   base class FileSR"
 * d80fe84f04be06db746ad0694ef90b4b0f1f86b5: "CP-6697: Fix FileSR capability
   config and param docs"
 * 8eb8126f0e02284776b0989dd5c4e51eb446f941: "CP-6697: Add "nosubdir" flag for
   NFSSR"
 * 3437a417b03ac6456ee9deab732c7b37e3980aa1: "CP-6697: Add "coalesce" flag to
   SR other-config"

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>