qemu-iotests: convert `pwd` and $(pwd) to $PWD

qemu-iotests: convert `pwd` and $(pwd) to $PWD

POSIX requires $PWD to be reliable, and we expect all
shells used by qemu scripts to be relatively close to
POSIX.  Thus, it is smarter to avoid forking the pwd
executable for something that is already available in
the environment.

So replace it with the following:

sed -i 's/\(`pwd`\|\$(pwd)\)/$PWD/g' $(git grep -l pwd)

Then delete a pointless line assigning PWD to itself.

Cc: kwolf@redhat.com
Cc: mreitz@redhat.com
Cc: eblake@redhat.com
Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Message-Id: <20181024094051.4470-2-maozhongyi@cmss.chinamobile.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: touch up commit message, reorder series, tweak a couple more files]
Signed-off-by: Eric Blake <eblake@redhat.com>