block/file-win32: add reopen handlers

block/file-win32: add reopen handlers

Make 'qemu-img commit' work on Windows.

Command 'commit' requires reopening backing file in RW mode. So,
add reopen prepare/commit/abort handlers and change dwShareMode
for CreateFile call in order to allow further read/write reopening.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/418

Suggested-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Viktor Prutyanov <viktor.prutyanov@phystech.edu>
Tested-by: Helge Konetzka <hk@zapateado.de>
Message-Id: <20210825173625.19415-1-viktor.prutyanov@phystech.edu>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>