Add qerror message if the 'change' target filename can't be opened

Add qerror message if the 'change' target filename can't be opened

Currently when using the change command to switch the file in the cd drive
the command doesn't complain if the file doesn't exit or can't be opened
and the drive keeps the existing image.  This patch adds a qerror_report
call to print a message out indicating the failure.  This error message
can be used to catch failures.

Current behavior:

QEMU 0.12.50 monitor - type 'help' for more information
(qemu) info block
ide0-hd0: type=hd removable=0 file=/dev/null ro=0 drv=host_device encrypted=0
ide1-cd0: type=cdrom removable=1 locked=0 [not inserted]
floppy0: type=floppy removable=1 locked=0 [not inserted]
sd0: type=floppy removable=1 locked=0 [not inserted]
(qemu) change ide1-cd0 /home/rharper/work/isos/Fedora-9-i386-DVD.iso
(qemu) info block
ide0-hd0: type=hd removable=0 file=/dev/null ro=0 drv=host_device encrypted=0
ide1-cd0: type=cdrom removable=1 locked=0
file=/home/rharper/work/isos/Fedora-9-i386-DVD.iso ro=0 drv=raw encrypted=0
floppy0: type=floppy removable=1 locked=0 [not inserted]
sd0: type=floppy removable=1 locked=0 [not inserted]
(qemu) change ide1-cd0 /tmp/non_existent_file.iso
(qemu) info block
ide0-hd0: type=hd removable=0 file=/dev/null ro=0 drv=host_device encrypted=0
ide1-cd0: type=cdrom removable=1 locked=0 [not inserted]
floppy0: type=floppy removable=1 locked=0 [not inserted]
sd0: type=floppy removable=1 locked=0 [not inserted]
(qemu)

With patch:
QEMU 0.12.50 monitor - type 'help' for more information
(qemu) change ide1-cd0 /tmp/non_existent_file.iso
Could not open '/tmp/non_existent_file.iso'
(qemu)

Signed-off-by: Ryan Harper <ryanh@us.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>