imsm: introduce helpers to manage file descriptors

imsm: introduce helpers to manage file descriptors

To avoid direct comparisions define dedicated inlines.
This patch propagates them in super-intel.c. They are declared globally
for future usage outside IMSM.

Additionally, it adds fd check in save_backup_imsm() to remove
code vulnerability and simplifies targets array implementation.

It also propagates pr_vrb() macro instead if (verbose) condidtion.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>