qemu-common.h: Document cutils.c string functions

Add documentation comments for various utility string functions
which we have implemented in util/cutils.c:
 pstrcpy()
 strpadcpy()
 pstrcat()
 strstart()
 stristart()
 qemu_strnlen()
 qemu_strsep()

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>