kvmtool: 9p: fix path traversal vulnerabilities

A path traversal exists because the guest can send "../" sequences to
the host 9p handlers. To fix this vulnerability, we ensure that path
components sent by the guest don't contain "../" sequences.

Signed-off-by: G. Campana <gcampana+kvm@quarkslab.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>