vfio: replace CONFIG_HAVE_KVM with IS_ENABLED(CONFIG_KVM)

It is more accurate to Check if KVM is enabled, instead of having the
architecture say so.  Architectures always "have" KVM, so for example
checking CONFIG_HAVE_KVM in vfio code is pointless, but if KVM is disabled
in a specific build, there is no need for support code.

Alternatively, the #ifdefs could simply be deleted.  However, this
would add dead code.  For example, when KVM is disabled, there is no
need to include code in VFIO that uses symbol_get, as that symbol_get
would always fail.

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: x86@kernel.org
Cc: kbingham@kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>