virbitmap: Introduce virBitmapParseUnlimitedAllowEmpty()

virbitmap: Introduce virBitmapParseUnlimitedAllowEmpty()

Some sysfs files contain either string representation of a bitmap
or just a newline character. An example of such file is:
/sys/devices/system/cpu/isolated. Our current implementation of
virBitmapParseUnlimited() fails in the latter case,
unfortunately. Introduce a slightly modified version that accepts
empty files.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>