pc: Fix error reporting for -boot once
Commit
0ecdffbb created pc_boot_set() for use from monitor command
"boot_set", via qemu_boot_set(). pc_boot_set() reports errors to
cur_mon, which works fine for monitor code.
Commit
e0f084bf reused the function int reset handler
restore_boot_devices(). Use of cur_mon is problematic in that
context. For instance, the "Too many boot devices for PC" error for
"-boot order=abcdefgh,once=c" goes to the monitor instead of stderr.
The monitor may not even exist.
Fix by switching to qemu_error().