util: Fix gem5img when used to manually unmount a disk image.

util: Fix gem5img when used to manually unmount a disk image.

When unmounting a disk image manually using the
`gem5img umount mount_point` command, the operation can fail if the
process is unable to stat any of the mounts in the mount table. On
some systems this can occur even when running using sudo.

Added an exception check so any mount points that fail to stat will not
cause the whole script to terminate early.

Change-Id: I69cd2494ad0e8c989e19ecd8af8a811905cd6c09
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39897
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>