kvm switch default start failed

[lguohan]

Description
kvm vs switch default start failed.

error log

admin@sonic:~$ sudo zgrep syncd.sh /var/log/syslog*
/var/log/syslog.1:Jun 13 07:50:54.128779 sonic INFO syncd.sh[3011]: Traceback (most recent call last):
/var/log/syslog.1:Jun 13 07:50:54.129229 sonic INFO syncd.sh[3011]:   File "/usr/local/bin/sonic-cfggen", line 276, in <module>
/var/log/syslog.1:Jun 13 07:50:54.129631 sonic INFO syncd.sh[3011]:     main()
/var/log/syslog.1:Jun 13 07:50:54.129961 sonic INFO syncd.sh[3011]:   File "/usr/local/bin/sonic-cfggen", line 208, in main
/var/log/syslog.1:Jun 13 07:50:54.130224 sonic INFO syncd.sh[3011]:     deep_update(data, parse_xml(minigraph, platform))
/var/log/syslog.1:Jun 13 07:50:54.130650 sonic INFO syncd.sh[3011]:   File "/usr/local/lib/python2.7/dist-packages/minigraph.py", line 417, in parse_xml
/var/log/syslog.1:Jun 13 07:50:54.130969 sonic INFO syncd.sh[3011]:     root = ET.parse(filename).getroot()
/var/log/syslog.1:Jun 13 07:50:54.131247 sonic INFO syncd.sh[3011]:   File "src/lxml/lxml.etree.pyx", line 3427, in lxml.etree.parse (src/lxml/lxml.etree.c:81110)
/var/log/syslog.1:Jun 13 07:50:54.131576 sonic INFO syncd.sh[3011]:   File "src/lxml/parser.pxi", line 1811, in lxml.etree._parseDocument (src/lxml/lxml.etree.c:117841)
/var/log/syslog.1:Jun 13 07:50:54.137421 sonic INFO syncd.sh[3011]:   File "src/lxml/parser.pxi", line 1837, in lxml.etree._parseDocumentFromURL (src/lxml/lxml.etree.c:118188)
/var/log/syslog.1:Jun 13 07:50:54.137880 sonic INFO syncd.sh[3011]:   File "src/lxml/parser.pxi", line 1741, in lxml.etree._parseDocFromFile (src/lxml/lxml.etree.c:117100)
/var/log/syslog.1:Jun 13 07:50:54.138191 sonic INFO syncd.sh[3011]:   File "src/lxml/parser.pxi", line 1138, in lxml.etree._BaseParser._parseDocFromFile (src/lxml/lxml.etree.c:111646)
/var/log/syslog.1:Jun 13 07:50:54.138662 sonic INFO syncd.sh[3011]:   File "src/lxml/parser.pxi", line 595, in lxml.etree._ParserContext._handleParseResultDoc (src/lxml/lxml.etree.c:105102)
/var/log/syslog.1:Jun 13 07:50:54.144880 sonic INFO syncd.sh[3011]:   File "src/lxml/parser.pxi", line 706, in lxml.etree._handleParseResult (src/lxml/lxml.etree.c:106810)
/var/log/syslog.1:Jun 13 07:50:54.145305 sonic INFO syncd.sh[3011]:   File "src/lxml/parser.pxi", line 633, in lxml.etree._raiseParseError (src/lxml/lxml.etree.c:105621)
/var/log/syslog.1:Jun 13 07:50:54.145726 sonic INFO syncd.sh[3011]: IOError: Error reading file '/etc/sonic/minigraph.xml': failed to load external entity "/etc/sonic/minigraph.xml"
/var/log/syslog.1:Jun 13 07:50:54.415647 sonic INFO syncd.sh[3011]: Creating new syncd container with HWSKU Force10-S6000
/var/log/syslog.1:Jun 13 07:50:54.764778 sonic INFO syncd.sh[3011]: bc7c23b08bf8be11ba3a4fbac998144aa20119fff7681bd3a3403ca37ab2d4d5
/var/log/syslog.1:Jun 13 07:50:56.049308 sonic INFO syncd.sh[3011]: syncd

The bug is introduced in #2924

Steps to reproduce the issue:
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Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

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