block: Fix VM size field width in snapshot dump

block: Fix VM size field width in snapshot dump

When printing the snapshot list (e.g. with qemu-img snapshot -l), the VM
size field is only seven characters wide.  As of de38b5005e9, this is
not necessarily sufficient: We generally print three digits, and this
may require a decimal point.  Also, the unit field grew from something
as plain as "M" to " MiB".  This means that number and unit may take up
eight characters in total; but we also want spaces in front.

Considering previously the maximum width was four characters and the
field width was chosen to be three characters wider, let us adjust the
field width to be eleven now.

Fixes: de38b5005e946aa3714963ea4c501e279e7d3666
Buglink: https://bugs.launchpad.net/qemu/+bug/1859989
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200117105859.241818-2-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>