KVM: selftests: Move KVM_FEP macro into common library header

Move the KVM_FEP definition, a.k.a. the KVM force emulation prefix, into
processor.h so that it can be used for other tests besides the MSR filter
test.

Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-26-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>