KVM: x86: Fold WRMSR fastpath helpers into the main handler

KVM: x86: Fold WRMSR fastpath helpers into the main handler

Fold the per-MSR WRMSR fastpath helpers into the main handler now that the
IPI path in particular is relatively tiny.  In addition to eliminating a
decent amount of boilerplate, this removes the ugly -errno/1/0 => bool
conversion (which is "necessitated" by kvm_x2apic_icr_write_fast()).

Opportunistically drop the comment about IPIs, as the purpose of the
fastpath is hopefully self-evident, and _if_ it needs more documentation,
the documentation (and rules!) should be placed in a more central location.

No functional change intended.

Link: https://lore.kernel.org/r/20250805190526.1453366-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>