Check block driver read error in pflash_cfi0x

If a flash file of size smaller than the flash size is specified in
the -pflash option, the block driver returns error. But the
pflash_cfi0x ignores the error. This results in a flash content of all
zeroes. And the simulation aborts while executing code.

This patch adds the checks for errors from bdrv_read and escalates it
to the calling code.

Signed-off-by: Vijay Kumar B. <vijaykumar@bravegnu.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
