libxl: avoid freeing an uninitialised bitmap

libxl: avoid freeing an uninitialised bitmap

If vm->def->cputune.nvcpupin is 0 in libxlDomainSetVcpuAffinities (as
seems to be the case on arm) then the VIR_FREE after cleanup: would be
operating on an uninitialised pointer in map.map.

Fix this by using libxl_bitmap_init and libxl_bitmap_dispose in the
appropriate places (like VIR_FREE, libxl_bitmap_dispose is also
idempotent, so there is no double free on exit from the loop).

libxl_bitmap_dispose is slightly preferable since it also sets
map.size back to 0, avoiding a potential source of confusion.

This fixes the crashes we've been seeing in the Xen automated tests on
ARM.

I had a glance at the handful of other users of libxl_bitmap and none
of them looked to have a similar issue.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>