Fix portstat parser to accommodate new reminder message in portstat output

[SuvarnaMeenakshi]

Description of PR

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 201911
• 202012
• 202205

Approach

What is the motivation for this PR?

portstat/test_portstat.py::test_portstat_clear fails with error:

tmp_ok_cnt = before_portstat[intf]['rx_ok'].replace(',','')
>           rx_ok_before = int(0 if tmp_ok_cnt == 'N/A' else tmp_ok_cnt)
E           ValueError: invalid literal for int() with base 10: ''

How did you do it?

With the recent change in portstat cli sonic-net/sonic-utilities#2466, the output of portstat shows a reminder in multi-asic platforms.
This is causing additional lines to be parsed in portstat output.
This change is to avoid parsing the reminder line or empty line in the portstat output.

How did you verify/test it?

Test case passes on multi-asic Linecard after this fix.

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[wangxin]

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trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
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tests/common/portstat_utilities.py:69:25: E261 at least two spaces before inline comment

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[wenyiz2021]

can we change this to:
for line in content_lines[separation_line_number+1:reminder_line_number-1]: ?

there are 3 lines I added to the print, 1st and 3rd are 2 empty lines, 2ns line is the Reminder print, the original error failed at the 1st empty line. In this case we don't need line 69-70 then

[wenyiz2021]

(Pdb) content_lines[separation_line_number+1:reminder_line_number]
...
u'Ethernet280 X 0 0.00 B/s 0.00% 0 0 0 0 0.00 B/s 0.00% 0 0 0', u'']

the last empty line will be ignored

[SuvarnaMeenakshi]

I intentionally did not do "[separation_line_number+1:reminder_line_number-1]" to keep it generic for future.
In future release if a Reminder is added without an additional empty line, then "[separation_line_number+1:reminder_line_number-1]" will miss the last line.
Or in case someone adds an additional empty line also.
So I thought we could parse the output from first line after header till line before "Reminder" and skip any empty EOF lines seen in between

[wenyiz2021]

ok, that makes sense to me

[wenyiz2021]

thanks @SuvarnaMeenakshi for this fix, just added a minor comment

[wenyiz2021]

LGTM