This commit addresses a part of the same problem as PR kata-containers#7623 did for the
golang runtime.  So far we've been rounding up individual containers'
vCPU requests and then summing them up which can lead to allocation of
excess vCPUs as described in the mentioned PR's cover letter.  We address
this by reversing the order of operations, we sum the (possibly fractional)
container requests and only then round up the total.

We also align runtime-rs's behaviour with runtime-go in that we now
include the default vcpu request from the config file ('default_vcpu')
in the total.

We diverge from PR kata-containers#7623 in that `default_vcpu` is still treated as an
integer (this will be a topic of a separate commit), and that this
implementation avoids relying on 32-bit floating point arithmetic as there
are some potential problems with using f32.  For instance, some numbers
commonly used in decimal, notably all of single-decimal-digit numbers
0.1, 0.2 .. 0.9 except 0.5, are periodic in binary and thus fundamentally
not representable exactly.  Arithmetics performed on such numbers can lead
to surprising results, e.g. adding 0.1 ten times gives 1.0000001, not 1,
and taking a ceil() results in 2, clearly a wrong answer in vcpu
allocation.

So instead, we take advantage of the fact that container requests happen
to be expressed as a quota/period fraction so we can sum up quotas,
fundamentally integral numbers (possibly fractional only due to the need
to rewrite them with a common denominator) with much less danger of
precision loss.

Signed-off-by: Pavel Mores <pmores@redhat.com>