centos, rhel: Fix DPDK preference requirements

[lyarwood]

What this PR does / why we need it:

These preferences actually require at least 4 vCPUs through their use of a SocketsCoresThreads spread ratio of 1:4:2 [1].

Functional tests asserting that we can create VMs using these preferences and instance types providing the required amount of resource are also tweaked to use a smaller size.

[1] https://kubevirt.io/user-guide/user_workloads/instancetypes/#spreadoptions

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Release note:

NONE

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[kubevirt-bot]

@lyarwood: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-common-instancetypes-kubevirt-functest	d6e74a6	link	true	/test pull-common-instancetypes-kubevirt-functest

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[lyarwood]

/hold

Ah actually the ratio is the issue here, 1*4*2 is 8 vCPUs, we should really be using a ratio of 2 for 1*2*2 4 vCPUs