Stop GParted hanging when non-named device is hung

This is to fix issue #131 (closed) "GParted hangs when non-named device is hung"

Quoting the detailed description from the first commit message:

A user reported that GParted would hang at "scanning all devices...",
when a fully working disk was named on the command line, but another
device on the machine was hung.

This can be replicated like this:
(on Ubuntu 20.04 LTS for it's NBD support)

1. Export and import NBD:
    # truncate -s 1G /tmp/disk-1G.img
    # nbd-server -C /dev/null 9000 /tmp/disk-1G.img
    # nbd-client localhost 9000 /dev/nbd0

2. Hang the NBD server and therefore /dev/nbd0:
    # killall -STOP nbd-server

3. Run GParted:
    $ gparted /dev/sda

Tracing GParted shows that execution of blkid never returns.

    # strace -f -tt -q -bexecve -eexecve /usr/sbin/gpartedbin 2>&1 1> /dev/null | fgrep -v ENOENT
    ...
    [pid 37823] 13:56:24.814139 execve("/usr/sbin/mkudffs", ["mkudffs", "--help"], 0x55e2a3f2d230 /* 20 vars */ <detached ...>
    [pid 37814] 13:56:24.829246 --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=37823, si_uid=0, si_status=1, si_utime=0, si_stime=0} ---
    [pid 37825] 13:56:25.376796 execve("/usr/sbin/blkid", ["blkid", "-v"], 0x55e2a3f2d230 /* 20 vars */ <detached ...>
    [pid 37824] 13:56:25.380824 --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=37825, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
    [pid 37826] 13:56:25.402512 execve("/usr/sbin/blkid", ["blkid"], 0x55e2a3f2d230 /* 20 vars */ <detached ...>

Tracing of blkid shows that it hangs on either the open of or first
read from /dev/nbd0.

    # strace blkid
    ...
    lstat("/dev", {st_mode=S_IFDIR|0755, st_size=4560, ...}) = 0
    lstat("/dev/nbd0", {st_mode=S_IFBLK|0660, st_rdev=makedev(0x2b, 0), ...}) = 0
    stat("/dev/nbd0", {st_mode=S_IFBLK|0660, st_rdev=makedev(0x2b, 0), ...}) = 0
    lstat("/dev", {st_mode=S_IFDIR|0755, st_size=4560, ...}) = 0
    lstat("/dev/nbd0", {st_mode=S_IFBLK|0660, st_rdev=makedev(0x2b, 0), ...}) = 0
    access("/dev/nbd0", F_OK)               = 0
    stat("/dev/nbd0", {st_mode=S_IFBLK|0660, st_rdev=makedev(0x2b, 0), ...}) = 0
    openat(AT_FDCWD, "/sys/dev/block/43:0", O_RDONLY|O_CLOEXEC) = 4
    openat(4, "dm/uuid", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
    close(4)                                = 0
    openat(AT_FDCWD, "/dev/nbd0", O_RDONLY|O_CLOEXEC

Clean up:

1. Resume NBD server:
    # killall -CONT nbd-server

2. Delete NBD setup:
    # nbd-client -d /dev/nbd0
    # killall nbd-server
    # rm /tmp/disk-1G.img

Going to fix this by making GParted specify the device and partition
names that it is interested in to blkid, rather than letting blkid scan
and report all block devices.  Do this both when GParted determines the
devices for itself and when they are named on the command line.