Prevent having more devices with the same name

[vhotspur]

Following should raise an error and second device should not be added.

echo -e "add dr4kcpu one\nadd rwm one 0x0\ndumpdev" | ./msim

Instead, dumpdev prints the following:

[  name  ] [  type  ] [ parameters...
one        dr4kcpu    R4000
one        rwm        [Start    ] [Size      ] [Type]
00000000000            0 none