Documentation: virt: Clean up paravirt_ops doc

Clarify language. Clean up grammar. Hyphenate some words.

Change "low-ops" to "low-level" since "low-ops" isn't defined or even
mentioned anywhere else in the kernel source tree.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230610054310.6242-1-rdunlap@infradead.org