provide a stub version of kvm-all.c if !CONFIG_KVM

This allows limited use of kvm functions (which will return ENOSYS)
even in once-compiled modules.  The patch also improves a bit the error
messages for KVM initialization.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[blauwirbel@gmail.com: fixed Win32 build]
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
