coding_style: add point about 0x in trace-events

Rationale: The // form is valid in C99, so this is purely a matter of

consistency of style. The checkpatch script will warn you about this.

8. trace-events style

8.1 0x prefix

In trace-events files, use a '0x' prefix to specify hex numbers, as in:

some_trace(unsigned x, uint64_t y) "x 0x%x y 0x" PRIx64

An exception is made for groups of numbers that are hexadecimal by

convention and separated by the symbols '.', '/', ':', or ' ' (such as

PCI bus id):

another_trace(int cssid, int ssid, int dev_num) "bus id: %x.%x.%04x"

However, you can use '0x' for such groups if you want. Anyway, be sure that

it is obvious that numbers are in hex, ex.:

data_dump(uint8_t c1, uint8_t c2, uint8_t c3) "bytes (in hex): %02x %02x %02x"

Rationale: hex numbers are hard to read in logs when there is no 0x prefix,

especially when (occasionally) the representation doesn't contain any letters

and especially in one line with other decimal numbers. Number groups are allowed

to not use '0x' because for some things notations like %x.%x.%x are used not

only in Qemu. Also dumping raw data bytes with '0x' is less readable.

8.2 '#' printf flag

Do not use printf flag '#', like '%#x'.

Rationale: there are two ways to add a '0x' prefix to printed number: '0x%...'

and '%#...'. For consistency the only one way should be used. Arguments for

'0x%' are:

 - it is more popular

 - '%#' omits the 0x for the value 0 which makes output inconsistent