KVM: selftests: Adjust number of files rlimit for all "standard" VMs

Move the max vCPUs test's RLIMIT_NOFILE adjustments to common code, and
use the new helper to adjust the resource limit for non-barebones VMs by
default.  x86's recalc_apic_map_test creates 512 vCPUs, and a future
change will open the binary stats fd for all vCPUs, which will put the
recalc APIC test above some distros' default limit of 1024.

Link: https://lore.kernel.org/r/20250111005049.1247555-8-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>