kvm, emulator: Rename VendorSpecific flag

Call it EmulateOnUD which is exactly what we're trying to do with
vendor-specific instructions.

Rename ->only_vendor_specific_insn to something shorter, while at it.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>