vm: mmap support

vm: mmap support

	. test74 for mmap functionality
	. vm: add a mem_file memory type that specifies an mmap()ped
	  memory range, backed by a file
	. add fdref, an object that keeps track of FD references within
	  VM per process and so knows how to de-duplicate the use of FD's
	  by various mmap()ped ranges; there can be many more than there can
	  be FD's
	. turned off for now, enable with 'filemap=1' as boot option

Change-Id: I640b1126cdaa522a0560301cf6732b7661555672