util: Introduce virBitmapShrink

util: Introduce virBitmapShrink

Sometimes the size of the bitmap matters and it might not be guessed correctly
when parsing from some type of input.  For example virBitmapNewData() has Byte
granularity, virBitmapNewString() has nibble granularity and so on.
virBitmapParseUnlimited() can be tricked into creating huge bitmap that's not
needed (e.g.: "0-2,^99999999").  This function provides a way to shrink the
bitmap.  It is not supposed to free any memory.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>