cmd: host: return value of do_host_bind()

When trying to bind to a non-existent file the following output is
displayed:

=> host bind 0 non_existent
Failed to access host backing file 'non_existent'
exit not allowed from main input shell.

The last line is quite unexpected and due to an incorrect return value.

If do_host_bind() fails, return CMD_RET_FAILURE (= 1).

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>