qemu: command: align disk serial check to schema

qemu: command: align disk serial check to schema

Disk serial schema has extra '.+' allowed characters in comparison
with check in code. Looks like there is no reason for that as qemu
allows any character AFAIK for serial. This discrepancy is originated
in commit id '85d15b51' where the ability to add serial was added.

Alter the disk-serial test to add a disk with all the possible
characters listed as the serial value.