virfile: Introduce virFileReadValueBitmapAllowEmpty()

virfile: Introduce virFileReadValueBitmapAllowEmpty()

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
virFileReadValueBitmap() 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>