kvm tools: Copy net/9p/9p.h

Header could not be included directly because among some minor
issues, the original header declared the same function twice:
int p9_errstr2errno(char *errstr, int len);
int p9_errstr2errno(char *, int);

A patch has been sent to 9P maintainers, this header should
be removed once the patch is in.

Until then, use a modified copy of the header.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>