Add memory_region_init_ram_from_fd()

Add a new function to initialize a RAM memory region with a file
descriptor to be mmap-ed.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170602141229.15326-5-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
