vkvmenu: Filter values with spaces not possible from bookmarks

[martinwe-adfinis]

Having an entry like this:

my-kv/some/path pass user=foo bar

Selecting the entry with vkvmenu results in the following error output (and failure):

Invalid filter (must be of form key=value): bar
Run with -h for help.
Failed to get secret.

Putting quotes around it like this does not help either (quotes have no special meaning):

my-kv/some/path pass user="foo bar"

Invalid filter (must be of form key=value): bar"
Run with -h for help.
Failed to get secret.

This limits vkv's filtering/checking capabilities, but also is confusing to users who expect the bookmark entries to behave identically to invoking vkv directly.