OvmfPkg/VirtioFsDxe: add helper for appending and sanitizing paths

OvmfPkg/VirtioFsDxe: add helper for appending and sanitizing paths

EFI_FILE_PROTOCOL.Open() -- for opening files -- and
EFI_FILE_PROTOCOL.SetInfo() --  for renaming files -- will require us to
append a relative UEFI pathname to an absolute base pathname. In turn,
components of the resultant pathnames will have to be sent to virtiofsd,
which does not consume UEFI-style pathnames.

We're going to maintain the base pathnames in canonical POSIX format:
- absolute (starts with "/"),
- dot (.) and dot-dot (..) components resolved/removed,
- uses forward slashes,
- sequences of slashes collapsed,
- printable ASCII character set,
- CHAR8 encoding,
- no trailing slash except for the root directory itself,
- length at most VIRTIO_FS_MAX_PATHNAME_LENGTH.

Add a helper function that can append a UEFI pathname to such a base
pathname, and produce the result in conformance with the same invariants.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3097
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201216211125.19496-17-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>