KVM: nVMX: Hardcode dummy EPTP used for early nested consistency checks

KVM: nVMX: Hardcode dummy EPTP used for early nested consistency checks

Hardcode the dummy EPTP used for "early" consistency checks as there's no
need to use 5-level EPT based on the guest.MAXPHYADDR (the EPTP just needs
to be valid, it's never truly consumed).

This will allow breaking construct_eptp()'s dependency on having access to
the vCPU, which in turn will (much further in the future) allow for eliding
per-root TLB flushes when a vCPU is migrated between pCPUs (a flush is
need if and only if that particular pCPU hasn't already flushed the vCPU's
roots).

Link: https://lore.kernel.org/r/20250919005955.1366256-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>