KVM: Allow building irqbypass.ko as as module when kvm.ko is a module

Convert HAVE_KVM_IRQ_BYPASS into a tristate so that selecting
IRQ_BYPASS_MANAGER follows KVM={m,y}, i.e. doesn't force irqbypass.ko to
be built-in.

Note, PPC allows building KVM as a module, but selects HAVE_KVM_IRQ_BYPASS
from a boolean Kconfig, i.e. KVM PPC unnecessarily forces irqbpass.ko to
be built-in.  But that flaw is a longstanding PPC specific issue.

Fixes: 61df71ee ("kvm: move "select IRQ_BYPASS_MANAGER" to common code")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250315024623.2363994-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>