bitmap: add a generic bitmap and bitops library

Add most used bitmap and bitops functions into bitmap.c and bitops.c.
Theses functions are mostly copied from Linux kernel source.

Some of these functions are already redefined in the VNC server. Some
of them could be used for some block stuff. The yet yo be submitted
NUMA work also need bitmaps.

bitops_ffsl() and bitops_flsl() are here because bitops/bitmap works
on unsigned long, not int, and we can't use current code because:
* ffs only works on int
* qemu_fls only works on int
* ffsl is a GNU extension

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>