qemu-img: Fix Column Width and Improve Formatting in snapshot list

qemu-img: Fix Column Width and Improve Formatting in snapshot list

When running the command `qemu-img snapshot -l SNAPSHOT` the output of
VM_CLOCK (measures the offset between host and VM clock) cannot to
accommodate values in the order of thousands (4-digit).

This line [1] hints on the problem. Additionally, the column width for
the VM_CLOCK field was reduced from 15 to 13 spaces in commit b39847a5
in line [2], resulting in a shortage of space.

[1]:
https://gitlab.com/qemu-project/qemu/-/blob/master/block/qapi.c?ref_type=heads#L753
[2]:
https://gitlab.com/qemu-project/qemu/-/blob/master/block/qapi.c?ref_type=heads#L763

This patch restores the column width to 15 spaces and makes adjustments
to the affected iotests accordingly. Furthermore, addresses a potential
source
of confusion by removing whitespace in column headers. Example, VM CLOCK
is modified to VM_CLOCK. Additionally a '--' symbol is introduced when
ICOUNT returns no output for clarity.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2062
Fixes: b39847a50553 ("migration: introduce icount field for snapshots")
Signed-off-by: Abhiram Tilak <atp.exp@gmail.com>
Message-ID: <20240123050354.22152-2-atp.exp@gmail.com>
[kwolf: Fixed up qemu-iotests 261 and 286]
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>