hostmem: Add hostmem-epc as a backend for SGX EPC

hostmem: Add hostmem-epc as a backend for SGX EPC

EPC (Enclave Page Cahe) is a specialized type of memory used by Intel
SGX (Software Guard Extensions).  The SDM desribes EPC as:

    The Enclave Page Cache (EPC) is the secure storage used to store
    enclave pages when they are a part of an executing enclave. For an
    EPC page, hardware performs additional access control checks to
    restrict access to the page. After the current page access checks
    and translations are performed, the hardware checks that the EPC
    page is accessible to the program currently executing. Generally an
    EPC page is only accessed by the owner of the executing enclave or
    an instruction which is setting up an EPC page.

Because of its unique requirements, Linux manages EPC separately from
normal memory.  Similar to memfd, the device /dev/sgx_vepc can be
opened to obtain a file descriptor which can in turn be used to mmap()
EPC memory.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-3-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>