fix --vgdb-prefix no / character interpreted differently by V gdbsrv and vgdb
When --vgdb-prefix contains no / character,
the Valgrind gdbsrv is interpreting the prefix value as the filename
prefix in the current directory, while vgdb interprets this as
a directory to be opened in the current directory.

Cbange vgdb.c so that it uses the same interpretation as V gdbsrv



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13485 a5019735-40e9-0310-863c-91ae7b9d1cf9