vmxnet3: show vmxnet3 with filtering capability [VPP-1452]

show vmxnet3 desc may display 5000 lines of output since it has 5 tables. Each
table may have 1000 entries. It would not be very useful to debug problem.

We need filtering capability for the subject show command. We need to be able
to display the descriptor table per interface, per interface per table, and
per interface per table per slot. The latter is the most useful.

tested the following valid combinations
show vmxnet3
show vmxnet3 desc
show vmxnet3 vmxnet3-0/13/0/0
show vmxnet3 vmxnet3-0/13/0/0 desc
show vmxnet3 vmxnet3-0/13/0/0 rx-comp
show vmxnet3 vmxnet3-0/13/0/0 rx-comp 1
show vmxnet3 vmxnet3-0/13/0/0 tx-comp
show vmxnet3 vmxnet3-0/13/0/0 tx-comp 1
show vmxnet3 vmxnet3-0/13/0/0 rx-desc-0
show vmxnet3 vmxnet3-0/13/0/0 rx-desc-0 1
show vmxnet3 vmxnet3-0/13/0/0 rx-desc-1
show vmxnet3 vmxnet3-0/13/0/0 rx-desc-1 1
show vmxnet3 vmxnet3-0/13/0/0 tx-desc
show vmxnet3 vmxnet3-0/13/0/0 tx-desc 1

negative tests and command is rejected
show vmxnet3 abc
show vmxnet3 desc abc
show vmxnet3 vmxnet3-0/13/0/0 abc
show vmxnet3 vmxnet3-0/13/0/0 desc abc
show vmxnet3 vmxnet3-0/13/0/0 rx-comp abc
show vmxnet3 vmxnet3-0/13/0/0 rx-comp 1 abc

Change-Id: I0ff233413496e58236f8fb4a94e493494c20c5cb
Signed-off-by: Steven <sluong@cisco.com>