Check error returns from posix_fallocate (RHBZ#579664).

Check error returns from posix_fallocate (RHBZ#579664).

posix_fallocate has a non-standard way to return error indications.
Thus all our calls to posix_fallocate were effectively unchecked.  For
example:

  $ guestfish alloc test.img 1P
  $ echo $?
  0
  $ ll test.img
  -rw-rw-r--. 1 rjones rjones 0 2010-04-06 11:02 test.img
  $ rm test.img

With this change, errors are detected and reported properly:

  $ ./fish/guestfish alloc test.img 1P
  fallocate: File too large

This is a fix for:
https://bugzilla.redhat.com/show_bug.cgi?id=579664