KVM: selftests: Add "delete" testcase to set_memory_region_test

Add a testcase for deleting memslots while the guest is running.
Like the "move" testcase, this is x86_64-only as it relies on MMIO
happening when a non-existent memslot is encountered.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200410231707.7128-7-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>