pmem2: move reservation struct to vm_reservation.c

src/libpmem2/map_posix.c

src/libpmem2/map_windows.c

src/libpmem2/vm_reservation.c

src/libpmem2/vm_reservation.h

src/libpmem2/vm_reservation_windows.c

src/test/pmem2_vm_reservation/TESTS.py

src/test/pmem2_vm_reservation/pmem2_vm_reservation.c