doc: grammify "allows to"

doc: grammify "allows to"

English language grammar does not allow usage
of the word "allows" directly followed by an
infinitive, declaring constructs like "something
allows to do somestuff" un-grammatical.  Often
it is possible to just insert "one" between "allows"
and "to" to make the construct grammatical, but
usually it is better to re-phrase the statement.

This patch tries to fix 4 examples of "allows to"
usage in qemu doc, but does not address comments
in the code with similar constructs.  It also adds
missing "the" in the same line.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>