qemu-img: Use only string options in img_open_opts

img_open_opts() takes a QemuOpts and converts them to a QDict, so all
values therein are strings.  Then it may try to call qdict_get_bool(),
however, which will fail with a segmentation fault every time:

$ ./qemu-img info -U --image-opts \
    driver=file,filename=/dev/null,force-share=off
[1]    27869 segmentation fault (core dumped)  ./qemu-img info -U
--image-opts driver=file,filename=/dev/null,force-share=off

Fix this by using qdict_get_str() and comparing the value as a string.
Also, when adding a force-share value to the QDict, add it as a string
so it fits the rest of the dict.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180502202051.15493-3-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>